Overwriting bare repositories' master

Hello,

I'm trying to setup a git hosting facility, such as repo.or.cz. The facility provides a pre-initialized git repository only accessible through git-shell.

The goal is to minimise the system admins' intervention, and I have a question about a branch 'overwriting'. For example, let's say the user makes an initial import to refs/heads/master for testing purposes, then wants to start over and import the real project. Can he put a wholy different git repository in place of the other one, at the same destination?

I tried and I found something that doesn't seem to follow the documentation:

repo_one$ git push Beuc@cvs.sv.gnu.org:/srv/git/sources/administration.git \
  master:refs/heads/master
# [OK]
repo_two$ git push --force Beuc@cvs.sv.gnu.org:/srv/git/administration.git \
  +refs/heads/master:refs/heads/master
updating 'refs/heads/master'
  from ee3bda653dfabaf0f78f2a9977abec180f2b19dc
  to   c9a726b610bafc82142a16af80b83d28375ca619
Generating pack...
Done counting 0 objects.
Total 0, written 0 (delta 0), reused 0 (delta 0)
Unpacking 0 objects
error: denying non-fast forward; you should pull first

From man git-push: "If the optional plus + is used, the remote ref is updated even if it does not result in a fast forward update."

This also makes one wonder how the 'pu' git branch is updated.

One the one hand, this means that sysadmin intervention is required to reset such a repository, which is bad. One the other hand, this is also a security because users cannot erase history, even if there a cron job to prune&pack the git repositories, which is good.

Is this by design? Or should it work?