If you have several documents from SharePoint Workspace (Office Uploader) that need to be deleted – there is no way to easily delete the all out – you actually have to do them one by one. But there is this workaround:
1) Open the Task Manager
2) Click on the Process tab
3) End these processes "msosync.exe" and "msouc.exe"
4) Open Run and go to c:\users\(username)\AppData\Local\Microsoft\Office\14.0\ (for Win 7 users)
5) Delete the folder OfficeFileCache
Restart SharePoint Workspace and your files will be deleted.