A cockblocker is the often stigmatizing label given to an individual who, under social norms, is charged with and found guilty of one or more of the following acts (the range of acts is not limited to those mentioned herein):
1) She/He interferes when two people are flirting.
2) She/He interferes when two people are being intimate by way of kissing, dancing, or cuddling
3) She/He interferes while two people are engaging in genital intimacy, foreplay, or sexual intercourse
4) She/He interferes in any of the above acts that involve more than two people.
Normally a cock blocker is only labeled such when she/he interferes with the prospect of sexual intercourse, but since any of the above can lead to sexual intercourse, we can expand our definition.
1) Ron (aware of his actions) walked over to Jim and Sally's table while they were having a romantic dinner on their anniversary. Ron's a cockblocker.
2) James purposely coughed loudly to interrupt Jenine's and Michale's goodbye kiss. James is a cockblocker.
3) Tim walked over to Gregory and noticed that his hand was in Christiana's pants. He inconsiderately bumps into Tim, making him feel uncomfortable and thus pull his hand out. Tim is guilty of cockblocking.
4) Kathy hid all the toys her parents were planning to use in tonight's secret orgy at the Krusty Krab. Kathy is a cockblocker