If your the IAP is non-consumable the easiest way would be to add a flag to NSUserDefaults
that becomes enabled once a user has bought the IAP. Then before you call the ad check if the flag is enabled or not.
Some sudo code for this would be:
// in whatever class handles your IAP
-(void)userBoughtOrRestoredTheNoAdsIAP {
[defaults setBOOL:YES forKey:HideAdsForUser];
}
// in your view controller
-(void)displayAd {
if (![defaults boolForKey:HideAdsForUser]) {
[RevMobAd showFullscreen];
}
}