I would have a discussion with the manager if he is holding HIPAA or SOX related data on that workstation.
And I would find out what the policy is on acceptable usage. As soon as he violates it, record the incident, follow the procedure (usually it is notifying IT management and HR and legal that an acceptable usage violation has occured and to protect company assets you are deactivating his network user and machine account. Then turn off his network port.)
If they are not willing to deal with the issue, notify your boss that all issues concerning this workstation will no longer be handled by you, since you cannot effectively administer the workstation. This means that management will be solely responsible for all legal ramifications of the loss of HIPAA or SOX data, inadvertant release of client data, etc.
Then when bozo user gets a virus or corrupts his system and needs help, point him to your boss and politely say, "I am sorry. I do not manage your workstation. You will need to discuss that issue with my manager."
Usually taking a hard line on the issue is the ONLY way to deal with these political games that happen in some offices. If you state clearly that you refuse to play them and management has to deal with it, the games usually quickly go away.