You can choose to use a lower caster level for spells, but not lower than the caster level at which you could first cast that level of spell. So a 10th-level wizard could cast fireball with caster level 5th for 5d6 fire damage, but could not go as low as caster level 1st for 1d6 fire damage, because they do not get access to 3rd-level spells like fireball until 5th level.
The actual rule for this is
You can cast a spell at a lower caster level than normal, but the caster level you choose must be high enough for you to cast the spell in question, and all level-dependent features must be based on the same caster level.
(PFSRD → Magic → Caster Level)
Note that usually there is no reason to do this—a caster level 5th fireball requires the same 3rd-level spell slot that a caster level 10th one does, so why wouldn’t you get the 10d6 fire damage? However, there is an important case where it does matter, and you do want to use the lowest caster level possible: crafting magic items. The cost of many types of magic item (potions, scrolls, staffs, wands, etc.) is proportional to the caster level, so higher caster level means higher cost. As a result, for most spells, people use the lowest possible caster level in order to save as much money as they can. For spells that really need their caster level—most offensive spells—including that caster level in the item gets very expensive, which is often a very good reason to not bother putting those spells in magic items. Many spellcasters get items with more “utility” spells that don’t care about caster level too much, and then use their own spell slots for combat spells that need all the caster level they can get.
Spell-like abilities also use a caster level, and the rules say
In all other ways [aside from components], a spell-like ability functions just like a spell.
(PFSRD → Magic → Spell-like Abilities)
So a kineticist could also choose to use a lower caster level with kinetic blast. There really is no great reason to do that, though, since you can’t make a wand or whatever of kinetic blast. Wanting to avoid lethal damage is plausible, but if the plan is to knock someone out, the kineticist should just go ahead and deal full, lethal damage—while someone else is dealing nonlethal damage. Kinetic blast is very weak, so there is little risk of accidentally killing your target, and other allies can have a far easier time dealing nonlethal damage (e.g. anyone using unarmed strikes can do so freely, any other melee attack can do so just by taking a −4 penalty, etc.).
If nonlethal situations come up a lot in your campaign, though, consider asking your GM about expanding the kineticist’s options. A merciful blast infusion is a fairly obvious possibility, though frankly I wouldn’t even force a kineticist to waste an infusion on it. I’d probably just allow the usual −4 penalty to work. For that matter, it may already work for kinetic blade—and you should have kinetic blade, it’s the one way kineticist actually works.
Just some other kinds of similar abilities to intentionally de-power one’s effects.
Another thing worth bringing up here is “undercasting,” a feature of certain psychic (occult) spells. Basically, psychics and the like can choose to cast certain spells they know at lower spell level (not caster level), allowing them to use a lower-level spell slot. For example, if one knows mind thrust III, which uses a 3rd-level spell slot, one can use a 1st-level spell slot to cast mind thrust I, even if they don’t otherwise know mind thrust I. There are no arcane or divine spells that support undercasting—if an oracle or sorcerer knows summon monster III but not summon monster I, they can’t cast summon monster I and they have to use a 3rd-level spell slot for summon monster III.
It’s third-party,2 but psionics has kind of the inverse of undercasting—augmentation allows more power points to be used (akin to using higher-level spell slots) to improve the effect of psionic powers. However, damage and the like doesn’t scale with manifester level (caster level equivalent) in psionics—instead it scales with power points, so your 10d6 fireball requires the equivalent of a 5th-level spell slot, instead of just being a 3rd-level spell slot cast by a 10th-level wizard. So psionic characters, unlike spellcasters, have very good reason to try to be economical about how much power goes into their effects. They are also bound by the same minimums that spellcasters are, however.
For other types of magic, you’ll have to check that particular kind of magic. Most of them probably won’t actually specify, so you’ll have to check with your GM. For instance, I’m not aware of any rule about this for supernatural abilities, and for many supernatural abilities, it may not make sense.
Technically, it’s not actually really defined that you need a certain caster level to cast a given spell—a wizard gets 3rd-level spell slots at 5th level, at which point they also happen to have caster level 5th, but nothing in the rules text actually says that the caster level 5th is necessary to cast fireball. Community consensus is pretty clear on how this should work out, and comparison with e.g. the costs of magic items lends some evidence for it.
And, disclaimer, I have worked for that third-party publisher, Dreamscarred Press.