Add a popup menu allowing to refuse a key sent to you.
Make accept the default so player who want it only have to hit return.
make this behavior an option in options
Refuse all keys by default
Accept all keys by default
Ask me when i am sent a key

-1 to adding any more popup messages, especially in-flight.

Why not make it so the key received message says which key you were just given so you can go delete it if you don't want it?

well if you ticked the option refuse all keys or accept all keys by default you don't have the popup menus. Personally i don't care about having a popup or not as long as i can refuse keys by default.

I already have a plugin that can easily be modified to do this.

-1 because that might lead to only being able to give keys to people currently logged in, and that would suck. Plus I hate pop-ups. Delete keys with extreme prejudice. problem solved.

+1 to ARF, and also add who sent the key:
"Conflict Diamond sent you a USER key: "Denic Margaritaville, Latos I-8"