К счастью для Миранды есть антиспам-плагины. Пора изучать.
Среди плагинов есть и такая милашка:
Sends spam (IM!) messages back to (IM!)
It is thought as a kind of fun, to give the (IM!) spamers back what they produce. ;-)
It is just there so you are able to think that you can do something about something you can't do anything about. ;-)
...And it is thought as a small demonstration of what "Spam Filter" could do in your plugin. ;-D