Re: uninstalling plugin and removing the corresponding mysql tables
Most plugins do not remove data automatically on deactivation. That’d be a bad thing, since you might occasionally need to deactivate for upgrade/testing purposes, and you wouldn’t want to lose your data.
Some plugins provide a manual data deletion/uninstall function. If the plugin creates a Dashboard panel, check the panel for an Uninstall or Remove Data button.
BuddyPress does not have such a button. If you uninstall BP and want to get rid of the database clutter, just delete all the tables that start with wp_bp_. BP only keeps a few pieces of metadata in the sitewide and blog-specific tables, not really enough to worry yourself about.