Forrester,
I don't think the troll-form is all that bad, even if one ignores the RP aspects.
First, if we're dealing with "munchkin" fighters, they probably have an 18 or 20 base strength anyway. So a bump up to 23 isn't so overpowering.
Second, if you add the -2 disorientation penalty and the -1 size penalty, a high-STR fighter's attack bonus might even be *worse* than when he started.
Third, the disorientation penalty applies to saving throws and ability checks, too. Buffing up your fighter while weakening his will save is not a great combo in my book.
So for an 18 Str fighter, you've got -1 to hit, +2 damage, +6 AC, -2 to all saves, and 10' reach. In my opinion that's good (particularly the reach) but not overpowering. However, if you wanted to tone things down more, you could always houserule that equipment does not instantly expand or morph to handle large and exotic body types. So your troll-paladins would still be able to wield their holy longswords or whatever, but they would be running around naked (no armor, cloaks, boots... and if you really wanted to be mean, no rings). Anyway, that's just a humble suggestion. The other option is to keep whapping your troll-PCs with holds, charm monsters, dominates, etc. until they wise up. This second option, at least, could be kind of fun.