You should still try to get an admin opinion, but really you have full rights to fire upon him and take your ore back. He is committing an illegal offense and self defense includes both protecting your well being AND property. That gives you the right to fire on him within the rules. As for his actions, there is nothing against the rules that he has done.
My advice would be to take two different steps - maybe three.
1.) Alert the law enforcement authorities of the house you were mining in. For this purpose, consider Onega-7 to be under Rheinlands influence, and Tau-23 to be in the interest of both Gallia and IMG. Furthermore, contact the official faction for the ID he was using. If he was Universal ID'd, you'd contact USI for example. More often than not, those factions will try to work with you to contact that person and resolve the situation. The law enforcement agency may contact them themselves, so you may be able to skip this part.
2.) Make a public announcement to all other miners in that area/house as to this person's name, affiliation, and ship class, with a recommendation not to do businesss with him. This will hurt him the most, as it hurts his pockets.
3.) Put a bounty on his head until he repays. If you really want merca to be interested, you can put in a condition that if the transport is carrying the ore, the Merc can force him to drop it and take it themselves instead of destroying him. Mercs would be very interested if you allowed them to do this, as it potentially gives them the ability to make a large amount of money should they catch him.
Also this isn't flood, so +1'ing isn't okay. Consider this to be a mod notice, I'm on my phone, so no pink coloration.