Block is Grindr's "No thank you" function. A blocked guy will not be able to contact you, and you will not appear in each other's main views (even if you're a favorite). Your chat history with the guy will be erased.
Tap the blocks (top right for Android, bottom right for iOS) in a guy's profile to block him.
How do I unblock a guy? Guys can not be unblocked individually. You'll need to unblock everyone in this instance. Follow the instructions below.
1. Open Grindr
2. Tap Menu > Settings > Privacy
3. Tap Unblock Everyone > Confirm