You can clone with
HTTPS or Subversion.
When I try to delete a remote branch, I get the following error (in the magit-process buffer):
$ git --no-pager push origin :refs/heads/somebranch
ssh_askpass: exec(/usr/bin/ssh-askpass): No such file or directory
Permission denied (publickey).
fatal: The remote end hung up unexpectedly
However, I can delete the branch with the command git push origin :somebranch.
git push origin :somebranch
There seem to be some configuration problem: your ssh, when called from git seem to not find ssh-askpass. Could you try to see if git push origin :somebranch work when called from emacs (using eshell for exemple)?
Closing because this is most likely a configuration issue.