Right now, only staff can remove content from threads. I don't know if you're able to change the date when the poll closes by yourself (I'll go check for the sake of curiosity on my alt in a minute) but either way just PM a relevant staff member and they'll handle it for you. Although it's probably best the poll be closed than removed so that people can see for the sake of curiosity what was voted for.
edit: just tried it and I wasn't able to edit my own poll without being a moderator. So yeah, just send someone a PM I guess.
Ummm... you know, sometimes I get the concern for potential abuse, but in the case of members removing their own polls, I really don't see why it would be a concern or why it should even matter?It'd work in theory but people could edit votes to sway in a certain favour, and the fact that not even any of us can add extra poll options when it's posted. It'd be okay to change poll timeout/close the poll but otherwise deleting them would probably not be easy given it has to be done through modcp for us, and the fact that well, members can't delete their own threads, so :( I just feel like it could be abused if a poll isn't going their way so they'll just remove it completely if that was possible to do. I mean not necessarily in all cases, of course, but there's the risk of that happening so maybe there could be a safeguard there of like a backup of a poll saved, kind of like edited posts where it saves backups?