poništiti

1. (decision, contract) to annul; to cancel; to revoke
   Sud je poništio ugovor. — The court annulled the contract.
   Izbori su poništeni na tri biračka mesta. — The election was annulled at three polling stations.
2. (of a ticket) to validate; to punch
   Nisam stigao da poništim kartu u autobusu. — I didn't manage to validate my ticket on the bus.
3. (of an effect, result) to cancel out; to negate
   Jedna greška je poništila sav naš trud. — One mistake negated all our effort.
4. (on a computer) to undo
   Slučajno sam obrisao tekst, pa sam poništio brisanje. — I accidentally deleted the text, so I undid it.

The imperfective is 'poništavati'.