Neither defeat or any variations of "defeat" is an option. As a DM, your job is to provide your players a good gaming experience.
That said, if you really want to be a wee mean about it... I can think of a few tricks to throw your beguiler off from his metaphorical horse

I have quite the experience in payback, actually. (Hey, nobody said I'm a perfect DM either, did they?) Using immune creatures isn't subtle enough, not to mention a very unfair move; taking away his trump card would make your beguiler's player feel useless. What follows are a few less evil, shady moves to achieve what you need.
1. solution: Make the Vow of Peace impractical in at least several situations! Since the aura affects all creatures, not just enemies, have the party group up with NPCs who are penalized by it via the plot, such as Barbarians, Bards or Dread Witches(Add extra extra hostile creatures or design sieges/ambushes so to stay at the appropriate CR). That will encourage him to think about using it at least.
2.: Curses, the taint of evil, afflictions, dismemberment... In short, permanent stat penalties. They won't make him unable to spam his aura, but they are enough to balance him until higher levels when the team's healer can patch him up.
3.: If I were you, I'd make use of irony. You said the beguiler has the Nymph's Kiss feat(I hope you are aware it doesn't give bonus to the DC of the Vow of Peace, just Charisma checks and saves; you phrased it as such, thus I felt I should point it out, just in case). Did he describe with whom he is/was in a close relationship with? It's not an actual requirement, but the Nymph's Kiss feat mentions the bonus comes from a maintained intimate affair with a fey. Why he has the benefits then? Because they never broke up, and the girl was waiting for the poor guy all this time of course! So throw his ex at him, hilarity ensues! It won't even be hard; they'll be justified to meet just about anywhere in the wilderness, depending on her race, and some circumstances can easily force them to group up. If he plays his cards smart, maybe they'll get together again; if he doesn't, sucks for him. And there you have a perfectly understandable plot twist to hide your revenge behind.
I'd come up with a less malicious significant other for him than a bitchy nymph or a clichic driad if he doesn't prefer those either, though. Female satyr are nice and shy in stark contrast with the males, for instance. A shy fey is still a fey, so you'd have to come up with a good excuse as to why is she even more withdrawn than average, unless you'd roleplay out a lot of unmentionables. They have... quite the drive.
4.: Grab your copy of Cityscape and paradigm shift your adventure into roleplay galore for a while! He will still make use of his aura(such as calming down rioters or panicked crowds), but no combat feat breaks a campaign low on combat.