If a user is deleted all his data is also removed from the forum DB. So the spammer can then sign up again and do everything all over again. Banning is almost always more effective than deleting against spammers.
Nothing prevents spammer from registering again under another name.
What prevents automatic spammers is reliable captcha.
If permanently banned users are an issue of any kind (DB, forum stats) they should be deleted (or excluded from statistics at least).