There is usually some unsolicited mail that has an already franked envelope enclosed, I put a load of other junk mail in it and post it off.
A taste of their own medicine and they are the ones paying for it.
I do the same. Anything that comes through the letterbox, that's hasn't my name on it, straight in the bin...to be fair, even the stuff with my name on it goes in the bin....I'll pay when I am good and ready