I always enter something along the lines of <expletive>@<expletive>.<expletive>. My hope is that they'll waste money on trying to email it and then get a bounceback that the email failed to be delivered.
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.