The problem as I see it is that you can't fight an irrational enemy rationally.
This is exactly the same dilemma that faces nations in the time of war. If you resort to an enemy's unethical, immoral or downright criminal methods it makes you as bad as they are. If you don't it's quite likely they will win so what are you to do? Either way you lose.
I'm not so worried about the legal aspects in this case. Unless I'm completely wrong about this, sending out these messages from your home PC is like saying to the spammers, "Here I am guys, come & get me".
