No, at least not by default for spells
D&D uses the Vancian magic system. One of the characteristics mentioned there is:
Magical effects are packaged into distinct spells; each spell has one fixed purpose. A spell that throws a ball of fire at an enemy just throws balls of fire, and generally cannot be "turned down" to light a cigarette, for instance.
(5e spells can be cast as long as you have spell slot, a bit different from the original Vancian magic, which is one use only per prepared spell.)
This is similar to what the Basic Rules say about what a spell is:
A spell is a discrete magical effect, a single shaping of the magical energies that suffuse the multiverse into a specific, limited expression.
You can then compare it to a computer program, and find that a spell is somewhat analogous to a computer program:
A computer program is a collection of instructions that performs a specific task when executed by a computer.
So by default, spells only do what they say they do. If the description says it can be toned down, it can. If the option is not provided, it can't be toned down.
Maybe your campaign is different
How magic works in your campaign might be different. Tell your intent to your DM and let them decide. These are starter ideas for your DM to legalize your intended outcome:
- Rule that it is possible to use magical effects outside spells. While spells do what they do, you can use non-spell magical effects.
- Modify all spells to allow the caster to control the damage.
- Give a feat or ability that allows the caster to control the effect of the spell, for free.
- In doing what you intend to do, you invent a new spell. It is not a cantrip yet (you can't reliably replicate it), or it is a cantrip but does not count against your normal cantrip limit.
Most DMs I've met usually don't bother too much and let you do it anyway, but some might prefer to run spells as written. If they hesitate to allow it because they can't find a rule that allows you to tone the effect down, you can mention those ideas.