How to hard reboot the Nexus 7 (2013) when touchscreen is dysfunctional?
I'm not 100% sure about the Nexus 7, but with my Nexus 4, you can hold the Power button for a very long time (Something like 20 seconds) and it will hard-shutdown. This should help facilitate a hard reset and let you get into Fastboot or Recovery mode if you need to.