1. To deprive another's vagina (aka box) of action. See cock block.
2. To prevent another from gaining access to the vagina. See also cock block.
3. In basketball, to prevent an opposing player from grabbing a rebound and scoring, by positioning oneself between the opponent and the basket.
1. Mary wanted to get with James at the party, but her ex-boyfriend showed up and boxed her out.
2. Joan was tired of Tom's advances, so she called her girlfriends over to box him out.
3. No one could ever box out Wilt Chamberlain.