Hi Paul
I imagine that deactivating a plugin for mobile users will not be the best approach for this. You probably want the symposium toolbar to stay active at all times but on the mobile version you would need to unhook certain aspects of it from WordPress.
Take a look http://codex.wordpress.org/Function_Reference/remove_action & http://codex.wordpress.org/Function_Reference/remove_filter - both of these functions can be used to rip out certain chunks of code from almost any plugin.