First off, by default, whenever you start a new presentation, your polls will automatically be set to "no responses". This is a setting which you can adjust in the Preferences section of the Settings screen.
Starting polls with no responses is useful if you present the same zeeting multiple times or want to run a test presentation but don't want your "test responses" to impact the actual presentation.
If you want to completely wipe a poll clear of it's existing responses, you can reset it entirely. To do this, go edit the poll, and then select the "Reset poll" button in the "Options" section of the poll.