Yes, in the majority of situations, if your adept has powers/abilities, they're simply active if they want them to be. According to the SR3 FAQ they can intentionally suppress/stage down their Killing Hands power, so there's no real reason they can't choose to suppress most of their abilities if they needed to, though I wouldn't let them do this with things that apply to their base attributes by default.
In reference to Richard's answer involving Magic Rating and limits on powers one can use, there's more detail to be had.
My digging revealed the following:
Source: 3E Core rulebook, page 168. ADEPT POWERS, paragraph 1.
An adept begins the game with a number Power Points equal to the
adepts Magic Attribute to "purchase" powers.
Source: 3E Core rulebook, page 168. ADEPT POWERS, paragraph 2.
An adept who loses Magic also loses a corresponding amount of powers,
so an adept who loses a Magic point must also lose one point worth of
powers. The adept’s player chooses which powers are lost. An adept
cannot have more levels in a power than the adept’s Magic Attribute.
For example, an adept with Magic 4 cannot have more than 4 points
worth of powers, none of which can have more than four levels.
When the core book was put out, but before MitS came along, there was the ability to purchase extra power points for karma. These did not increase your Magic Attribute, therefore (using the book example) you would be limited to no more than 4 power points worth of active functioning powers at any one time. Aside from invalidating the option to purchase power points and use an initiation system, there's no other change really noted about this.
Source: Magic in the Shadows, page 33.
Adepts and Gaesa
If an adept suffers Magic loss, the gamemaster can choose, to apply a
geas to 1 point worth of the adept’s powers. This offsets the loss of
the Magic Point and allows the adept to continue to use those powers,
so long as the geas is fulfilled.
This is your new loophole that lets an adept retain powers in excess of their actual Magic Attribute rating/power point limit.
A player can voluntarily take a geas on a power of an adept when
purchasing that power. Each power can have only one geas, but a
single geas can be applied to several powers. A power limited by a
voluntary geas costs 75 perfent of the standard coast (round fractions
up to the nearest quarter point), with a minimum cost of .25 Power
The example in this section used Astral Perception, a 2 point power, which could get for 1.5 points with a voluntary geas. In theory, you could have a PC who applied various limitations across all of their assorted powers, which would enable them to start a brand new adept with up to 8 Power Points worth of abilities (at 6 magic). Such a character would also have to restrict how many of their abilities were active at once, assuming the geasa were set up in such a way that conditions permitted them all to function in a given setting (like a night-time restruction).
Source: Shadowrun Third Edition FAQ (Frequently Asked Questions). Version 2.1a. Last Updated: December 01, 2006
A normal adept (not on the Magician’s Way), has these choices for
Raise Magic attribute by 1 + gain a power point + learn a new metamagic technique; or
Raise Magic attribute by 1 + gain a power point + alter astral signature; or
Shed a geas.
Shedding the geas here would have the nice benefit of returning you to your full un-restricted power use, regaining the magic point that was offset, but I have seen people who just focus on gaining the extra magic and power points, learning to live with their restrictions. Some of them have also simply 'given up' and refused to honor their geasa, permanently sacrificing that magic point. To discourage this, I make a point of letting them know that they will never be able to re-learn the powers tied to that point they have chosen to sacrifice. They turned their back on the magic, sometimes it's a critical power they need for future development.
Source: State of the Art 2064, page 68.
This focus is a variant of the power focus specifically designed (and
only of use to) adepts. It increased the bonded adept's effective
Magic Attribute by its Force. This does not grant adepts additional
power points, but does affect the amount they can have active at a
time, as well as affecting tests, ranges, etc that use the adept's
current Magic attribute. The adept focus also grants dice equal to
its Force for the adept's Drain Resistance Tests, but it does not
affect whether Drain is Stun or Physical.
Confirmation of total limit previously discussed.
An Infusion focus allows the adept bonded to it to tap into his inner
potential and activate a specific adept power that he does not
normally possess. An Infusion(Combat Sense 2) focus, for example,
would give the adept the power of Combat Sense 2 for as long as the
focus was bonded and active. Infusion foci are dedicated to a
specific power, and an adept who knows that power must participate in
the enchanting process. Likewise, when an adept bonds the focus,
another adept who knows the power must be present to teach him how to
use the focus to unleash it. The rating of the infusion focus must
be equal to the power's point cost x 2 (So a Combat Sense 2 Infusion
focus would be Rating 4). Powers gained by an Infusion focus count
towards the total number of powers an adept may have active at one
time. The Magical Power (p. 22, MITS) may not be activated through
use of an Infusion focus.
I made sure to go back and double-check, even looked for an errata on it. This one says you have a limit on the number of powers you may have active at one time, not your power points. I can't be certain this is where the confusion was introduced, but it does open a window of disagreement. If we just roll with the power points value, as that has been fairly well established at this time, the adept in the example above would need to dampen some of his pre-existing powers to use the focus, or be using it in conjunction with an Adept Focus of at least Rating 2.
In practice, if my players need to suppress or enable an ability that requires no drain/success tests, we let them do so as a free action. This gives them the ability to shut off one and enable another, while still being able to take actions in their given combat phase.