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.
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