If an account has been hacked, how do you know that the person who has assumed control of the account hasn't changed the email address or taken control of the email account?
Any action you might take still has the potential to end up in the hands of the hacker, so nothing would change.
The only solution I can see is ban the account with a message there is a problem with the account and they need to contact the board admin. If they do contact you, you will need to figure out a means to validate their identity.
Normal people… believe that if it ain’t broke, don’t fix it. Engineers believe that if it ain’t broke, it doesn’t have enough features yet. – Scott Adams