Can't turn off find my phone on ruined phone to get credit

My old phone fell into a port-a-Jon and recovered. It no longer works and was told by my carrier (Verizon) that if returned in any condition, I would receive my $200 credit toward my new phone. The Verizon store has tried to get "the find my phone" turned off on the old phone, with no luck. Now, they say it is an Apple problem. Can somebody give me direction on what to do here?