No, no, no. You don't do it like that. Now you've traded a potentially strong ally for an ordinary blacksmith, but still have the risk that he'll kill you some day. Either accept that risk and make him an ally, or pull up your big girl pants and kill him.
The best plan, though, isn't to befriend the hero with the special anti-you skill, but to befriend every other hero in the world. Make all of them a part of your retinue, as a special force to oppose other demon lords. Give them room and board, training, good equipment, even a regular wage. Make it public that you're doing this. Not only will the existence of this hero academy serve as a deterrent against other demon lords, but just in case another anti-you hero spawns it will attract them to your side as well, where you can easily finish them off in secret.