Remove Icloud

The account holder can sign out Apple ID from the account and delete the account from servers , the other person has no rights to remove the account ( as account credential identifications is required ) .

See the articles Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support

How to delete your Apple ID account - Apple Support

Note : Pease make sure don't try to take credentials from the user , you can only provide the method to them , even you try to contact apple support they will not help ( you can't speak to apple support on behalf of them and should never ) .

RAMSES2022 Said:

"Remove Icloud: Hello Dears, I have an Iphone 7 plus need to remove iCloud, Thanks"

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Removing an Apple ID:

Is this a used device?

What to Do:

A. Signing Out:

The original owner should have signed themselves out of it. So, learn form this, having the donator go to this site, to disassociate it from there own account: What to do before you Sell, Give Away, or Trade in your iPhone, iPad, or iPod touch - Apple Support

B. Unlocking This:

If you are locked, and you do not have a proof of purchase, then it cannot be locked. The locking of an iPhone is a security option. Go Here: If you Forgot the Passcode on your iPad, or your iPad is Disabled - Apple Support