Mmm... We had this very problem at our old RPing sites and for a time it gave us quite the headache, too.
*Looks over everything and sighs*
Taking a walk down memory lane:
First things first, never EVER tie anything to the post count. Seriously, people can and will spam up the forum. I know this because, well, I’ve done it before. About 5-6 years ago, me thinks, but at that time I had not yet come to appreciate the usage of words or the English language as I do now.
Secondly, at the SFC (when it was still active) we used to use a ranking system for Captains and Vice Captains.
Rookie VCs < Adept VC < Elite VC / Rookie Captain < (Very) Elite VC / Adept Captain < Elite Captain
Simple, yes?
Only the Elite VCs were allowed to possess a realistically underdeveloped to moderate Bankai and have the chance to challenge a Captain for their position. The ranks themselves were determined by the admins and mods upon the approval of an application depending on how active the person was, how much they’ve contributed to the site, and how well they could RP (so no, not everybody could magically become an Elite – jeez). If the member wanted to argue their position or up their rank, all they had to do was send a PM to the staff members, ask, and give a few reasons as to why they thought that they should have so-and-so rank (the rest who chose to publicize and dramatize it had their asses handed to them).
Despite what one might think this method ended up working out pretty well because our members were a lot more reluctant in kicking other people’s characters out of their positions lest they had gone completely inactive, have left the forum all-together, or it was for plot purposes. In the case of your forum, however, I think that setting up a limit somewhere might be a good idea.
Also, in the case of a challenge I personally do not believe that a Captain’s powers should be restricted lest they choose to impose that restriction upon themselves. Techniques, abilities, and kidou (even forbidden ones) are, after all, created to be USED one way or another. The Captains are the pillars of the Gotei and are deservingly so – if a VC wants to become a captain they should have to earn it through their own means may it involve brains and/or brawn.
…
… And that’s my two cents for the day. Me mind is lacking sugar at the moment so everything’s scrambled and I can’t remember what else I wanted to say. XP
*Goes to find her cookie stash* <(

<)
Edit: Oh yeah!
Creating an exp system can work, but then you have to make a guide to determine how much exp gets you what. Plus, there will be people who will complain and are strictly against the usage of basic math for some reason so that's really up to you guys.