Right click on My Computer on the workstation and select Properties.
Click on the Computer Name tab and then click the Change button. Select the workgroup radio button and set the workgroup to Workgroup. This will remove you from the domain. Enter the admin username and password and reboot.
The Server trust has now been reset.
Upon reboot, verify that the domain no longer sees the server as a member by logging into the Domain Controller (DC) and use Active Directory Users and Computers. Traverse down the Active Directory name and into the Computers folder. Look for the server. If it is still there delete the computer from the folder..
Once it is gone.... add it back into the active directory from the workstation reversing the process of removing.