If you no longer can access an iPhone or iPad you can remove them in Find My iPhone on iCloud.com. When you remove a device, it’s removed from your Devices list and Activation Lock is turned off.
To sign in to Find My iPhone, go to icloud.com/find. Please make sure you sign in with the same apple ID that's being used on the damaged device.
You need to remotely erase it before you can remove it from your account.
In Find My iPhone on iCloud.com, click All Devices and then select the device you want to erase.
If you don’t see All Devices, it’s because you’ve already selected a device. Click the name of the current device in the centre of the toolbar to access the Devices list and then select a new device.
Erase the device.
Because the device isn’t lost, don’t enter a phone number or message.
If the device is offline, the remote erase begins the next time it’s online. You receive an email when the device is erased.
When the device is erased, click Remove from Account.
All of your content is erased and your device is now ready to sell!