My Japanese translation would be a person who acts mean so that one cannot score. In these kind of phrases, English has plenty, but unfortunately, Japanese doesn't have much. The translation I proposed is less offensive, since it doesn't include the male genital.

My translation would be read as:
Saigo no issen wo koerarenaiyouni ijiwaru wo suru hito

By the way, the phrase you gave is a bit vague, since it could mean a lot of things. Please see below link:

http://www.urbandictionary.com/define.php?term=cock block

You'll see all the usages for cock block and basically it means preventing someone from intercourse, but it also uses scores, poontangs, or accidentally or intentionally, out of jealousy or anger.