You found out the right first step yourself: Block the source IP address. Sure it will turn into a game of whack-a-mole with them changing their IP but eventually, their customers will get fed up with their downtime.
Second idea: Javascript redirect all of your pages to your own subdomain. Again, its just a step in an arms race, but this would be a little too hard/expensive to take to court. You can win an arms race if you try.
Second idea: Javascript redirect all of your pages to your own subdomain. Again, its just a step in an arms race, but this would be a little too hard/expensive to take to court. You can win an arms race if you try.