People joining PF just to instantly leave

I've seen this happen a few times and I agree it can be annoying. I joined and left a PF a few times myself, because I ended up changing my mind and wanted to do something else. Nowadays though I think about if I actually wanna do said content before I join, so I won't be one of those people that leaves before the dungeon starts :P

I had someone joining then leaving instantly.
There was only one slot left and it seemed he did that to remove our party from the list.