The Monks add their wisdom modifier as a bonus to AC,
The Saint template, in the Book of Exalted Deeds, a template capable of being acquired by a player, among many other things adds the character's wisdom modifier as an insight bonus to AC.
I know i could simply disallow players from using this template, but another idea i had was simply changing the monk's bonus to AC into an insight bonus.
If your player manages to get a Saint status from his roleplaying, his exemplar behavior as a character and takes the Saint template, then you shouldn't worry about him having these two AC bonuses.
That's not such a big deal, especially for a monk who doesn't wear armor anyway.
I'd agree - if some one is THAT Good as a character, you have an excellent role-player to have earned the Saint template in play.
Yes that's fine, but then it becomes a problem for me whenever he takes the Vow of Poverty feat. Good role playing or not, that is simply way too much power for a single player.
Uh - ok, Vow of Poverty is relatively powerful, I agree.
However, there are VERY strict role-playing guidelines that go with both the template AND the vows. Also, one of the requirements to become a saint is a minimum of 3 Exalted feats. I presume you are already familiar with these requirements and the restrictions that they impose.
Also, he has NO gear beyond the minimums permitted by the Vow of Poverty. Nothing masterwork, no potions, no scrolls, no anything beyond a single set of clothes, simple weapons, a single sack and simple mundane gear. (As a druid, most of those are not even a consideration unless he cashiered his wild shape ability, which would be rather not-smart of him.) He cannot get a haversack, he cannot ever have a flying carpet, he cannot use the spiffy new +5 vorpal sickle from his share of the dragon's hoard. He gets a FULL share of treasure and it MUST be donated to NPCs. The Vow replaces all the goodies the character would ever even theoretically be able to acquire, costing him a great deal of flexibility.
As a druid expensive material spell components are not as much of an issue - but, he'd better have Eschew Materials as a feat or he's hosed. (Given how wild shape druids are probably going to select that feat anyway, along with Natural Spell, he's now burned off FIVE feats.) His only recourse to cast spells that require those are to burn XP (1 XP : 5 gp) or for the other characters to cough up those components. As a druid he's pretty much stuck with a club or staff, a sling and a pouch of stones for weapons. He cannot ever own magic items - not even to read scrolls or trigger wands or staves to benefit himself in any way.
You have to be strict about the restrictions of the Vow - your player's character has already burned 3 general feats at a minimum, and if at any time he violates the taboos of any of his Exalted feats, they ALL go away and cannot be gotten back. He has to walk a very fine line, and the poverty vow rewards it. And your player's character HAS to be strictly good as well - his fellow characters cannot be permitted to do a lot of things that many parties with more "flexible morality and ethics" will do.
However, the rewards are pretty fixed, and he'll burn through the bonus exalted feats by or before 20th level in a right hurry, resulting in a number of "what do I do with these feat slots now?" unless he just HAS to fill those slots. Ability Focus (Golden Ice) or something similar is about the only thing I can think of as acceptable once the rest of the good exalted feats have been snapped up.
EDIT: As a monk, he has even less flexibility - the Vow of Poverty can reasonably be interpreted to preclude the character from EVER wielding any weapon save the aforementioned sling and quarterstaff.
So what if he has a high AC? It's not like a monk even approaches the hitting power of almost all the other characters in a typical party, not from what I've seen. I know some of the other GM's on the boards - at least one has such a character in play and he finds it perfectly acceptable.
I play in a Forgotten realms campaign with druid1/monk7 character who has vow of poverty. Druid class is alternative version from PHB 2 which allows him to take wolf-shape at will (basically gives him more speed, more strenght and more AC). Ok, he has a really high AC (35 if I remember correctly), but his attack bonuses and damage output aren't so good. This monk has been a blast to play though. It's first character I've ever played which I can actually say is lawful good. His role is pretty much the protector of the group. The monk is always there when someone is about to get whacked, either taking hits from monsters or giving hit points to others with his stigmata ability. At first other characters hated him because of his strict moral, but he has saved their lives so many times that they are starting to respect him and act good because of choice, not because they're forced to. This monk having too much power? Not true in my opinion. In D&D it always comes down to damage and he just can't deliver it. Not even close to what other characters can deliver.
Now the Saint template... I looked at it, and while it gives great defensive abilities, it would basically drop his attack bonus by two, which isn't so great to start with. So while I think the monk could deserve the Saint template, I just don't think it's worth missing two levels.
I had a player with a Human Druid with Vow of Poverty (who later gained the Saint Template) in an old 3.5 campaign.
Not mentioning the old, broken Wild Shape, his most impressive (and hateful) ability was the Aura of Protection that Saints share with angelic creatures; basically, a full-movable Lesser Globe of Invulnerability + full-movable Magic Circle against Evil.
This was one of the most broken things I've ever seen in 3.x (all the other bonuses were good, but THIS was really too much).
Now, if we add on top of that the old, good, exceedingly broken Wild Shape ability...