Can't remove resource

We have all of our employee resource loaded at the root workspace then assigned down the workspaces to relevant projects.

Occasionally when importing projects (from Microsoft Project for example), their resource will also import and automatically be added to our root workspace. We don't want this as it creates duplicates or resources that don't follow our standard format.

However even when I delete the resource assignments from the original projects, I am unable to delete it from the workspaces as it says that it is being used in a child project. However I have checked and the resource isn't assigned to any projects.