remote does not support deleting refs

Hi,

I tried to setup a central git repository. Access is via ssh.
How do I delete a remote branch? If I try it using
 > git branch -d -r origin/testbranch
 > git push origin :refs/heads/testbranch
I get the error

To ssh://myserver.com/my/path/to/repository
  ! [rejected]        testbranch (remote does not support deleting refs)
error: failed to push some refs to 
'ssh://myserver.com/my/path/to/repository'

So how do I setup my remote repository to allow deleting remote branches?

Thanks, Martin