With OBSE there are a couple of options.
Without OBSE, you could set up some claws or something as a weapon to force the old weapon out of their hand. There's a way to make it so the player can't unequip the weapon...a script would have to do it.
You could also use a script to equip a weapon, and then unequip it. It would likely have to keep running in order to keep the player from equipping a weapon.
Or temporarily set their weapon skills to zero to discourage them from using a weapon.