How to disable voice chat in GTA V?
To turn off voice chat on PS3: Hold down Select then scroll down the Interaction Menu to Chat. Switch it over to whichever setting you prefer.
No-one, Crew, Friends, Crew + Friends, or Everyone
Note: Changing this setting can effect your ability to succeed in a Rally Race. Also, it appears that you cannot change this setting once you've joined a lobby. So you may want to consider enabling voice chat before starting a mission.