Not having the ThirdPersonActor is the "real" solution IMO. On the other hand, it might be a little bit more work. At least, unless you override some of the weapon/inventory methods I think you wind up getting some Accessed None's, so it winds up either being a little bit gross either way (cluttering the log with exceptions versus duplicating weapons/methods just to guard against accessing a null ThirdPersonActor).
So my opinion, if you want to make existing weapons disappear in third person, then shrinking the actor is probably the tactical option. If it's a custom weapon, then override so you don't have the third person actor at all.