Users who have disabled receiving PM's should not be able to send them either

I sometimes receive messages from users who have disabled receiving them themselves. This is annoying. If someone talks to me, I expect them to be willing to listen to any answers I might give too.

So, my suggestion is: if you disable receiving messages, then you should not be allowed to send them either. Sure, that will not stop people from enabling it temporarily just to fire off some messages only to disable it again afterwards, but it would be a start.