They are probably already subscribed (to test it works). Better to enter a duff@emailaddress.com which will mess up their stats.
One of the most important lessons of judo is that you never use force yourself – you just redirect the force of the other person so they hurt themselves.
Using that philosophy in life makes stuff a lot easier ;)
And, funnily, it fits here, too. Using their own newsletter to get it blacklisted. The more they send, the less are received.