Unfortunately there is no way to fix this. The developers of the game seem to have turned a blind eye to their customers in regards to that particular hack.
However, there is one way that you can get online to play multiplayer on the game. Have a friend send you a game invite. That seems to by-pass the issues you have with the negative exp.
Also be aware that you can report the players who did this using the dashboard reporting tools. Go to their bio and select File Complaint then follow the instructions. The PET will investigate and take the necessary actions.
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