Amelia aka Amelia Booking, WPAmelia, and Booking for Appointments and Events Calendar – Amelia
It leaves a bunch of tables in your WordPress database by default. You can clear them out with a simple click or 2.
If you have uninstalled Amelia, install and activate it.
under the Amelia menu go to Settings, then Activation and change the slider for the Delete Amelia content option. Now when you deactivate you should see a checkbox wit the option Delete tables, roles, files and settings once the Amelia plugin is deleted, Amelia cleans up most of its mess once deleted.
It leaves these tables amelia_events, amelia_providers_to_events and amelia_users with your database prefix in front of the names. You can manually delete these through phpMyAdmin if they bother you.