While I personally wouldn't do it due to liability, a look at CERT and other places were server security is discussed is always thought provoking... and good for increasing your whitehat skills. You should always try the normal channels (abuse@ etc) though many times it doens't work.

I understand your frustration, I deal with hundreds of SPAM a day. I wish some of these f***s were local so I could pay them a visit.

That being said, it's probably not worth it. (trying to DOS them that is.)

"To be civilized is to deny one's nature."