MaxineN: ... as an extra step delete/de-activate all iCloud accounts and start from fresh so they cannot be re-invited in.
As MaxineN said: If you don't want to buy a new device to be 100% sure, factory reset the iPhone & use a new iCloud account.
Behodar:... Especially if there are purchased apps, music etc. then you wouldn't want to delete the account.
The emotional freedom of not being stalked is worth the price of any app / music library!