I just deleted a test installation of Omeka but cannot delete the directory because there is a very stubborn .SER file located inside the HTML Purifier plug-in.
If I needed to uninstall the plug-ins before deleted, I guess it is too late for that. I also cannot change the permissions. I'm not sure exactly what an SER file is, but it must be important!
You can go into the database and "uninstall" the plugin. Go into phpmyadmin or some other database admin and find your database. Go to the
plugins table, then update the
active field for each plugin to "0" if you want that plugin deactivated.
This, of course, won't help you with the file permissions for that that folder. You'll probably need someone with root privileges to delete it, or update the permissions on that file to allow you to delete it.