[kelnos]

I would assume that a factory reset would take care of it. Otherwise you couldn't sell the phone to someone else and revoke your ability to find it.

[matt-attack]

You have to do a full iCloud logout which involves your iCloud password in order to disassociate a device from your Find My Phone. It's actually quite involved.

[schrodinger]

You can’t do a factory reset without the device password.

[Karunamon]

You can still reflash with DFU mode, but even then you can't activate the phone if FMI was turned on.