You cannot do this from the Modeler at the moment. What you can do, is install TortoiseSVN 1.6 (not 1.7!) and delete the branch via the repository browser. See this StackOverflow question for information on how to do this:
Deleting might sound scary but you do not ever really delete anything from subversion.
If you want this feature in the Modeler, I would suggest you file a feature request.