I've deleted my account before. Just to not be "in the database" for any future site emails, member to member contact attempts, or whatever might be intentionally done (or unintentionally done) with that site's data in the future. The sites had the extension for allowing users to delete their account, though.
But Jim's earlier recommendation presumably already covered that. Change your email address to
info@<insert relentless marketing company here>.com, and not only will you be unable to re-activate your account, but heaven help them if the board tries to contact you again...
