Its the same concept as a cock blockbut its a uni-sex word, meaning it can be used for girls and guys. Its a person that basically prevents a person from hooking up with another.
A group of three or more girls and a duff come to a party. The duff is named Theresa Marie. All night Theresa Marie will not stop talking to the girls and prevents them from talking to the guys. She is constantly taking them away from the party and is being a total crotch blocker to the guys