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Is there a method to delete the poll?
Referring to the information at
https://wiki.simplemachines.org/smf/Posting#Poll_Options, Moderator can delete polls.
-snip- All board moderators can modify polls and do the above actions, as well as view the results of the poll at any time while the poll is running. Moderators can also remove polls and lock voting.
So, the only thing you might try is to contact the Forum Staff or Moderator to delete the poll you created.
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Edit: if it is self-moderated, you can not also delete poll, but you can delete the whole thread. I have just tested that and I was not able to delete self-moderated poll unless the whole thread is deleted.
As far as I know, members cannot delete threads themselves. Maybe you mean what was deleted was the content of the thread?