Someone can sit in guarded and disarm to their heart's content if they are fast enough...
Some ideas regarding it...
Disarm could drop the disarmer's defense for a single round.
Disarm could not be able to be used unless you are neutral, offensive, or kamikaze... (If you are trying to protect yourself... you won't be looking for an opening to disarm...)
Disarm could make you lose TWO combat rounds.
Disarm could only be used after a succesful parry.
Disarm could only be used once in several rounds of combat. Or once in a single combative fight.
Disarm could give disarmee a free hit, or chance to hit. (Like a counter attack...)