Right click on any folder, select “Repo-browser” and find your branch on the left panel. From there, you can issue a “Delete” command directly on the repository:
As far as I know, you cannot issue a delete command on the top folder* of the working copy, probably because that would effectively destroy the working copy.
(*) You can of course remove the branch from the working copy if it’s an inner folder, i.e., you check out e.g. https://svn.example.com/repo/project/branches instead of https://svn.example.com/repo/project/branches/feature-blah.