To delete a local branch
git branch -d the_local_branch
To remove a remote branch (if you know what you are doing!)
git push origin :the_remote_branch
or simply use the new syntax (v1.7.0)
git push origin --delete the_remote_branch
If you get the error
error: unable to push to unqualified destination: the_remote_branch The destination refspec neither matches an existing ref on the remote nor begins with refs/, and we are unable to guess a prefix based on the source ref. error: failed to push some refs to 'git@repository_name'
perhaps someone else has already deleted the branch. Try to synchronize your branch list with
git fetch -p
The git manual says
-p, --prune After fetching, remove any remote-tracking branches which no longer exist on the remote.