I’m sorry about this problem, which can happen if the delete gets interrupted for some reason. It is something I intend to fix on the next release, but for your purposes, you can make a quick edit to the plugin that will allow it to complete the deletion.
In the plugin file PDb_init.php on line 348 and 349, you’ll see:
$sql = 'DROP TABLE ' . Participants_Db::$fields_table . ', ' . Participants_Db::$participants_table . ', ' . Participants_Db::$groups_table . ';';
$wpdb->query( $sql );
Just comment those lines out since the tables have already been deleted, like this:
// $sql = 'DROP TABLE ' . Participants_Db::$fields_table . ', ' . Participants_Db::$participants_table . ', ' . Participants_Db::$groups_table . ';';
// $wpdb->query( $sql );
The proceed with the plugin delete.