To "cock-block" someone. Mostly used in french, and although usually meant to describe a female who lafarges a man, the sexes can be interchanged. This means that the most common use would be for a woman who lafarges a man who is trying to "get with" either her or another female. It can however also mean a woman who lafarges another woman, a man who lafarges a woman, or a man who lafarges a man.
This word can be used to describe actions both with negative and positive results (i.e. if the dude was a big douche-bag, he can still be lafarged).
Also, lafarging someone can be intentional or accidental.
"Dude, the girl totaly just lafarged me. I was so close to hooking up with Cindy, but the lafargeuse came in and started talking about forest fires...MOOD KILLER!"