The solution is even easier. There is no need to have messages individually marked, the system knows what is being sent and can handle it automatically.

Auto detect spam messages. If a guy keeps sending out the same copy and paste, after 2 or 3 sends, just forward future copies to /dev/null.

Now this has to be personalized! If a women responds to those types of messages above some certain threshold, then hey, let her receive them, no problem.