Turns the wizard into a rich commoner. This is probably a bad thing when facing a demon that could snap you like a twig.
Only prevents creatures from getting close to you on a failed will save. The save DC of Repulsion is, what, 10+6+8 (assuming Int 26) = 24? The Balor's will save is +19. Unless it buffs with
Unholy Aura, the at-will SLA that gives it +4 to saves and AC, among other things. Then you're just screwed.
Or it could just cast
Blasphemy from a distance. You know, that at-will SLA it has that is devistating to anyone with significantly less hit dice than the caster level of the spell, which is 20 in this case.
Also, Balors have SR 28. Assuming Spell Penetration and Greater Spell Penetration, that's a +16 to your CL check to overcome SR, which is, what, a 45% chance to succeed?
Full round casting time, so the Balor gets to take his turn while you are waving your arms around. Also, I would very much be interested in hearing what monsters you would summon to deal with a Balor.
Geas/Quest (lesser extent)
Ten minute casting time. If you can't see the problem with using this spell on a Balor...
Also, SR 28.
Otiluke's Freezing Sphere
Does not do enough damage to drop a Balor in one casting. Blasphemy, on the other hand, will drop the wizard in one casting.
Also, SR 28.
Balor's fort save is +22. The DC of a 6th level spell, as calculated above, is about DC 24. Let's say you have Spell Focus and Greater Spell Focus. That's DC 26. Balor succeeds on a roll of a 4 or higher. Unless it buffs with
Unholy Aura, of course.
Also, SR 28.
Disintergrate (all from 6th spell level for Sorc/Wiz)
Not only is it at the same DC as Flesh to Stone, but it also requires a ranged touch attack. That's two chances to go wrong for the price of one! Well, three if we count SR.
In conclusion: No, you're not taking out a Balor with your level 12 spellcaster.