Please delete erroneously pushed branch from onionoo.git

Branch task-16907-2 should have been pushed to my public repository, but instead I pushed it to the official repository. Now I cannot delete it anymore:

~/src/onionoo$ git push origin --delete task-16907-2
remote: + refs/heads/task-16907-2 onionoo karsten DENIED by fallthru
remote: error: hook declined to update refs/heads/task-16907-2
To ssh://git@git-rw.torproject.org/onionoo.git
 ! [remote rejected] task-16907-2 (hook declined)
error: failed to push some refs to 'ssh://git@git-rw.torproject.org/onionoo.git'

Thanks in advance!