I've only just noticed, but if you check out your own profile, there's a 'delete this account' facility. Its a sort of hari kari button. Not all forums have this.
I would imagine that if you press it, then your history and you become a guest. I'm not sure how final a step it is, or whether if the ex member had a change of heart that account could be reactivated.
Of course, it would be easy to reactivate the individual but I'm not so sure about the account.