Where things sometimes get complicated is if you aren’t able to turn on your handset due to the damage sustained. However, you can still deactivate Find My iPhone, using iCloud.com on another device:
• Turn off your iPhone (or whichever device you wish to remove).
• Sign in to www.icloud.com/#find with your Apple ID. (If you can’t see Find My iPhone on iCloud.com, it is likely that your account just has access to iCloud web-only features. For access to other iCloud features, set up iCloud on your iPhone or iPad).
• Select the device you wish to remove from the account.
• If the device is offline, click Remove from Account.
• If the device is online, click Erase.
• Once this is complete, click Remove from Account.
If you are unable to access your iCloud, you can contact Apple here.