Is there a thread on the Assetto Corsa forums for the mod? If so I'd suggest contacting the creator and ask them to take it down. If they don't, post in the thread you've not approved the mod. The community there is quite good at quashing unapproved mods.
If it's not posted on the AC forums, you might try creating a thread there soliciting help finding the modder. Again the community there doesn't like this behaviour.
Good luck.