I did a check for "orphan" polls, polls that are not linked to any post, and found myself a few.
If you have MySQL >= 4.1 you might want to have a look if you can clean up a little too:
WHERE POST_ID NOT
This will show the poll questions of orphaned polls.
delete from ubbt_POLL_DATA where POST_ID not in (select POST_ID from ubbt_POSTS);
delete from ubbt_POLL_OPTIONS where POLL_ID not in (select POLL_ID from ubbt_POLL_DATA);
delete from ubbt_POLL_VOTES where POLL_ID not in (select POLL_ID from ubbt_POLL_DATA);
These 3 do the actual cleanup (polls, poll options and the votes). Change the ubbt_ prefix if you happen to have another one in use.