Hum, let's see. The "spaming" issue it's not related to the number of polls or threads, but the visibility of those in the front page of the forum and the way the system works. If i or anyone happens to stop a little at the forum and place 5-10 short posts in different threads, the front page suddenly becomes the "me" page. And that happens with everything and with everybody here at different times of the day: polls, art, opinions, technical advises, jokes, goodbyes and hellos, etc.
So, instead of trying to put hancups or absurd limitations to the liberty of expression of other members, improve the view of the front page. It's pretty easy, really. Keep visible the last 3 posts in every forum section in the front page, so no topic or user can take over the whole front page at once, and all the sections of the forum keep their visibility and show their latest updates.
If you are determined to go after the polls, allow the OP to place new polls inside the same thread.