Using topic branches and doing pull requests creates considerable clutter. Well documented here:
That article says that the solution is "auto close branch", but that only works when you're all working off the same repository. Otherwise auto-close doesn't seem to be available.
I can't work around this by having everyone in the same repository because there are no branch level permissions. See this bug report, where the "wontfix" rationale is that you can just use forks:
Fixing either issue would solve the problem as far as I'm concerned. Allow auto-close everywhere, or give us branch level permissions.