Tap the menu button at the top right corner and then tap close. Select a final options (more then one if you would like). This will close the poll, and no one else will be able to participate in the poll anymore. You can save the final date(s) directly to your connected calendar with the link below the final option and share the results with all the participants.
Tap re-open on the black bar at the bottom of you screen
Tap the menu button at the top right corner then tap re-open