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1.
A male or female who interferes with a female's attempts to get some sexual action. Similar to a cock blocker, except it's a vagina that's being blocked from action, hence the term "box."
"I would totally have picked up that guy if you douche bags hadn't been box blocking me all night."

"I like going out with the guys to watch the game, but I never get laid those nights since they're such box blockers."
by HNIC August 03, 2007
 
2.
One who prevents a female from engaging in consensual sexual intercourse. Can be a male or female, but usually it is the latter. Analagous to cockblocker. From the slang word box.
Mike was totally about to fuck me, but then Amy had to be a boxblocker and screw it all up.
by Chris Kilmer January 17, 2005