When you cast a spell that has an actual level given below its name, you choose (and expend) a slot equal to the spell's level or higher and the spell is cast at that level. This is all better explained on page 201 of the PHB. Some spells have additional effects that happen when you cast them using a higher level slot.
Cantrips however are a slightly different story; their spell level is 0, they don't expend spell slots and as such they can't be cast at a higher, or lower, level (PHB pg.201).
This means that there is no choosing the level at which it's cast because its spell level is always constant.
The extra effects gained by a cantrip aren't based on slot level but on your character level and are worded in such a way that doesn't give you a choice of whether they happen or not.
The spell creates more than one beam when you reach higher levels. (Eldritch Blast - cantrip)
This spell’s damage increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6). (Acid Splash - cantrip)
As opposed to spells that use slots:
At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, a target’s hit points increase by an additional 5 for each slot level above 2nd. (Aid - 2nd level spell)