Brent,
I'm afraid to say that isn't supported at this time. While you can remove the named users from your organization, if they're removed from the Workforce project prior to removing them from the organization, you'll receive error messages indicating there is a problem. I don't think you want to go this route, it's going to be disruptive to Dispatchers.
Another alternative; to remove the association between completed assignments and workers being removed. This will involve set the assignments.workerId field to null/empty and ensuring the workers you want to remove from the project are not associated with any assignments. You won't see who was assigned the work.
Neither of these are great options, and speak to the fact that we don't support this workflow. It may help to outline why you need to maintain the completed assignments in the Workforce project. I may have another alternative based on your use case.
Craig