Ideally: all buffs/debuffs spells, including future ones, have special keywords as arguments.
If, for example, the target already has active buff/debuff with X keyword, then ANY spells with the X keyword cannot be casted on the target (or can be casted, but won't work anyway) until this active buff/debuff with the X keyword ends. It will allow full control for all buffs.
Easy way out: if defence_multiplier in current target effects, then no spells with defence_multiplier can be casted. I'm not sure though how well it will work with items and traits, since they can give defence multipliers too.