It's a level 6 weapon, so reintroducing minimum requirement to use a weapon would most likely solve this problem. Increase costs for autoattacks to 1 AP per 2 level difference, rounded up. If you are two using it would need 4 AP for an autoattack, for level 2 the same. Or a more classic way: You need XX strength to be able to lift it and use is for any kind of attack/skill.
Of note, you'd want to increase the cost of the abilities that use the weapon as well. Otherwise, this sounds like an elegant solution.
In DoS 1 if you found a powerful staff early you could shoot the Magus Bolt for a ton of damage without a scaling cost. They did scale the melee/ranged weapon abilities based on weapon levels being too high though.
