Once a poll is running on your website, deleting a question may have a negative impact on the statistics shown for a given question. Also, deleting a question cannot be undone.
Open the poll the question is associated with by clicking on the title in the Poll Manager.
In the Poll Manager, hover over the question you wish to delete on the right-hand side. You'll see an "x" over the box that says "disassociate." Click on that to delete your question. This cannot be undone.