first you need to establish a local rule is an agreement to full extend. I don't think it is unless the other party explicitly confirms he agrees. implicit agreements or contracts aren't necessary valid, explicit ones are as long as they're not against the law (or forum rules I guess)

he has no reason to enforce private agreements. I'd bet most moderators wouldn't ban me even if I explicitly agreed to be banned if I post somewhere, unless I break some forum rules
a moderator's job is to enforce the forum's rules, not private agreements