Assuming this branch isn't an external or a symlink, removing the branch should be as simple as:
svn rm branches/< mybranch >
svn ci -m "message"
If you'd like to do this in the repository then update to remove it from your working copy you can do something like:
svn rm http://< myurl >/< myrepo >/branches/< mybranch >
Then run:
svn update