This is a suggestion for Plugins and Themes.
For plugins, on the page under "Action" where Activate/Edit is listed, for those plugins already deactivated, include a "Delete" Option. This would delete all of the Theme's files from the plugin directory, with a "Are you sure" splash window. The only way to delete plug in files now, that I'm aware of, is to use my ftp program to go in and manually delete it.
A similar thing could be done for theme files as well.