If you are using an accelerator with Revit Server and need to remove it, you will need to delete the environment variable since there is not a way natively in Revit to remove it.
If you are using the IMAGINiT Clarity addin you can remove it via the Tools dropdown on the IMAGINiT tab, Clarity panel. If you use this tool when you have a model open, the change will not take affect until you exit out of that session of Revit and start a new one.
How to Manually Remove the Environment Variable
Close out of Revit first.
Through Windows Explorer, right click on This PC and select Properties or through the Control Panel, select System
Select Advanced System Settings
Select Environment Variables
Locate the RSACCELERATOR201# that you want to remove, then select Delete
OK out of all dialog boxes