How to reset iPhone to factory settings?
Is it signed in to Find My iPhone? If so you can go to http://icloud.com, select your phone and click erase iPhone.
If you did not configure iCloud with find my phone you could try to let the battery drain (to switch it off) and then switch it on by
- pressing the home button
- connect it to a computer (USB)
When connected it will start in recovery mode and then you might be able to wipe it with iTunes.