Cockblocker

A cockblocker is a person who prevented his/her male friend from having sex by being really annoying or interupting the couple. Being a cockblocker might ruin your friendship by quite a bit, unless it's your best pal that might forgive you after a couple of days.

"Hey, what the hell was that for?!"
"What do you mean?"
"You god damn cockblocker, you ruined it all."