Is there an easy way to destroy the internal microphone hardware on iMac?
No guarantee [nor insurance] is ever going to cover intentional destruction &amp; in fact a fraudulent claim may be an invitation for a nice police visit, dependant on jurisdiction. 

Smacking things with hammers [at your own risk] usually prevents normal operation of delicate electronic devices, though.
Dismantling with a soldering iron might better preserve the surrounding structures.