Most spammers are stolen accounts. The owners of those accounts will eventually get them back. RMT spammers aren't out buying new copies of the game to spam with, that's too expensive. Instead, they steal accounts, strip their gold, and spam with the characters on it (or create new characters to spam with on other servers).

Deleting the characters on the accounts wouldn't really hurt the spammers, they get suspended eventually anyways, and will just start using another stolen account. It would only hurt the players who have had their accounts stolen.