You should be able to go to Workspaces and remove/add any mappings there directly.
- Go To Source Control Explorer
- Expand the Workspaces drop-down along the toolbar in the Source Control Explorer
- Click the last entry to manage workspaces
There you should be able to remove mappings.
Here is an article discussing Workspaces
http://www.woodwardweb.com/teamprise/000334.html