Blue
Ravenous Bugblatter Beast of Traal
It comes up. All. The. Time. Bless is a big culprit. It + paladin aura, all the time. Any magic weapon in the party + Bless, all the time. Haste + any magical defense, all the time. Sacred weapon + Bless + magical weapon, all the time.
Sounds like your problem may be Bless, not general stacking. That's a spell that many consider powerful and some say is overpowered. What if just it didn't stack.
If you still want nothign to stack, here's my thoughts. The design team reduced the various bonuses already (+3 max magic item instead of +5, Concentration, etc.) in order to keep them small enough to stack. If you want to keep the same math without allowing stacking, I'd change magic weapons to +1/+3/+5, and allow concentration to be maintained on two spells at once (since now buff spells will be much less useful).
If you don't want to keep the same math, well that's the difference right there. Monsters will outpace players vs. the base game, so will be tougher at higher levels.