The opposite of a cock block; when the girl is preventing you from scoring...
"God! Tina is being such a snatch block!"
A woman who interferes with another woman's attempts to score with a guy. Similar to some definitions of cock block, except that a woman is competing for the attention of another woman's man.
Girlfriend: My roommate Griselda is such a snatch block!
Boyfriend: What do you mean?
Girlfriend: Out of all my friends, she only talks to you. And she practically dresses up whenever you come over!