Deleting a workgroup

Network administrator shared files to my computer by adding my computer to his workgroup. I noticed my computer name under his workgroup on the network. I can see his printer, his workgroup name, and his computer names. Please help? Before he did that, my computer files and folders were private and he couldn’t see none of my files. He said now he can’t see my files but I am worried that is why i made it private. Please help me get rid of his workgroup and computer names from being seen on my computer.