You can leave Jams before they've ended, but I've had real-life get in the way and have had to abandon Jams midway or entirely.  Now, they're listed on my "Game Jams You've Joined" even though I didn't participate.

Please implement ability to delete these.

I would say that even if they were willing to implement that feature, it wouldn’t happen for quite a while.

An alternative route might be to appeal to support to remove your association with the Jam in question. Since in your case, there is no project - and no way for you to abuse the system somehow by doing this. That being said: Please only contact support about this (right now) if it really, really bugs you. Both the employees in support and the administrators are swamped with work right now.