In Git, the branch tag is removed, but the commit will remain. VSTS does not perform garbage collection on the server side, and commits will remain there and will be restored. The only way to really destroy the branch is to create a local clone, delete the repository on VSTS, create a new bare repository in VSTS (you can use the same name) and paste the contents again.
TFVC , destroy (tf destroy $/Project/Branch/To/Destroy), . , .