Your a smart person Lutz. You should be able to figure something out. Can you just give email address's to MODS and have them search for accounts using that email on here and if the emailing continues just search and start banning people?
Or can you block the person so you will not receive any emails from them? Yes it would take a long time but it would solve the problem i guess.
I am sure there are many other options you could do to stop this.