
For the record, this phrase is vulgar and will be offensive to some people.
I ran a couple of google searches. You can see the English word used as a noun or adjective on French language forums and chatboards... i.e., saying that someone is
un/une cockblock or calling someone
une copine cockblock. But in the grand scheme of things, the English word isn't used very frequently, and it seems that people generally have to ask what it means, so it isn't widely understood in French. I also saw it used as a verb a few times, with a Frenchified spelling:
cockbloquer
I have seen a couple translations used to explain what the action is:
cassage de coup, blocage de queue... but none of them seem to be actually used in and of themselves (only to define the English word)
. I also saw the person defined as
un obstacle.
In short, I couldn't find a true equivalent in French slang... but wait for the native speakers to help.