What about a user not allowing to send the same message that he/she sent... Like 3x-5x in a row? This would limit to send the same picture. And also when a user sends fast messages multiple times it would give this person a pop up reminder to slow down and it would give him/her a few second to send. Maybe 3-5 seconds. I think spamming would be slow and the spammer might get frustrated and stop spamming