If my Teemo Strategist ability triggers and I can only choose a unit with Deflect, can I choose not to use the ability?

Yes — you can get out of it, but not by skipping the trigger itself.

Teemo's defend trigger is mandatory (no "may"), so it goes on the Chain as a Pending Item and you must choose a legal target if one exists (Rule 402.4). However, choosing the Deflect unit incurs Deflect as a Mandatory Additional Cost — and at the Pay Costs step you may decline to pay it (Rule 404.2). If you decline, the ability is removed from the Chain and never finalizes: no cards revealed, no damage dealt.

So the practical result is the same as "not using the ability" — you just have to let the trigger appear and then refuse the Deflect cost. Note the reverse is also true: if there were no Deflect unit, you'd be forced to fire the ability.