I want my last paycheck or I am not returning the key.
They may not pay you till they get their key. Why risk it?
They are bringing this on themself.
I see how you want to use it as leverage, and they are wrong not to pay you. That being said they should have gotten the key back when you quit.
I didn't quit. They fired me for talking about how to get unemployment because of their failures to pay.
You need to talk to your ex employer like an adult and come to an agreement where you get paid and they get their key back.