Don't you just love when you want to message someone, then this shows up in the above image? Hey, why not indicate that the user disabled messaging on the user page like having no message button?
Oh, this would be very useful, many creators had disabled messaging. Spend 5-10 minutes for a message and than see that advice. This isn't enjoyable O.o
Yes! A notification would be very helpful. Just do the check before the user sends a message.