To block someone from hitting more marijuana, usually mid-rotation on a piece or bleezy or spliff. Similiar to a cockblock.
Potblock scenario Ronald was about to take his hit
Zackary says, "No I think you have had enough man." and sticks his hand in front of the piece, blunt, spliff.
Susan: I want to go lay out by the pool but there are two athletic girls out there showing off their bikini body's.
Spencer: You just got hotblocked girlfriend!
Susan: What's that?
Spencer: It's like getting cockblocked but the pool is like the person you want and the hot girls are like a big cock.