Nilisimama ili nivute sigara = I stopped (moving) (in order) to smoke.
Niliacha kuvuta sigara = I stopped/quit smoking.

This is why giving us single words as exercises is useless!

I get kusimama as "to stand (still), stop, be stationary, and kuacha as "quit, stop, cease".

So who knows? If you think your answer is a valid translation given what you have to work with, report it next time the sentence comes up. Worst that could happen is it doesn't get accepted.

I agree. It's unclear without context telling what or who is stopping.

Yes! Seconding this. This exercise would be much more helpful if the word was "Stop!" (rather than "to stop") with acceptable translations being "Acha!" or "Simama!"