Green’s Dictionary of Slang

cock block n.

[cock block v.]

1. (US black) a woman who permits sexual intimacy but refuses intercourse; or anyone who prevents others from pursuing sexual relations.

[US]Eble Campus Sl. Apr. 2: cockblock – interfere with or prevent two people from pursuing intimate relations: ‘He really cockblocked me last night.’ Also a noun, one who cockblocks.
[US]Ebonics Primer at [Internet] cock block Definition: female who fucks around to the point of intercourse and stops Example: I was chillin at this party when dis young cock block bitch licked once and left.

2. (UK black) a male who interferes with another male's sexual advances.

[UK]A. Wheatle Crongton Knights 180: ‘You fat ass has been in the way [...] Stop being a cock-block!’.