iskvariti

1. (+ acc.) to spoil; to ruin
   Kiša nam je iskvarila izlet. — The rain ruined our trip.
   Nemoj da mi iskvariš iznenađenje! — Don't spoil the surprise for me!
2. (of a person, morals) to corrupt; to deprave
   Novac ga je potpuno iskvario. — Money completely corrupted him.
   Loše društvo je iskvarilo tog dečka. — Bad company corrupted that boy.
3. (iskvariti se) to go bad; to take a turn for the worse
   Do podne se vreme potpuno iskvarilo. — By noon the weather had turned completely bad.
   Odnosi među njima su se iskvarili. — Relations between them went sour.

In everyday speech pokvariti is more common, especially for machines and devices breaking down.