If the users don't have any activity then most likely the only table where they exist is mp_Users. If they were created through the UI they may also have rows in mp_UserProperties, if they have ever logged in they may have rows in mp_UserLocation.

We don't have a way to bulk delete users form the UI other than when deleting a site from a multi site installation there is a site pre-delete event handler that can delete any feature specific data associated with the user.

Be carefull with your query and be sure to backup the db first. You could delete all users by siteid but you want to make sure you leave an admin user, so you would need to know the id of the site and the user to keep then it would be

DELETE FROM mp_Users WHERE SiteID = x and UserID <> y

Hope that helps,

Joe