With all the Bronze plugins now part of the download from WordPress.org you need to remove the Bronze plugin folders from your wp-content/plugins folder.
Do this via FTP, do not uninstall them via the admin dashboard (if uninstalling, tables in the database are removed!).
In your wp-content/plugins folder, using FTP delete all wp-symposium-***** folders EXCEPT wp-symposium – do not delete wp-symposium plugin folder.
When you next visit your plugin admin page you will be told that a number of plugins have been de-activated as they are no longer exist, that’s ok.