I don't see any way to enable stuff except for what is in the p2 UI. And I see no update stuff there at all - not even in the list of things to install.
p2 don't even seem to have enable/disable functionality, only install/uninstall.
Some plugins allow controlling their load-on-startup behavior. These will be listed in the preferences, under General → Startup and Shutdown. If the plugin provides view, you will need to close those views (in all perspectives) for this to work.