Deleting Forums Using Mysql

Deleting forum from drupal, only delete terms, if you want to delete all its nodes, comments and terms from database, truncate the follwing tables from Drupal Database.

truncate comments;
truncate content_type_forum;
truncate forum;
truncate node;
truncate node_revisions;
truncate term_data;
truncate term_hierarchy;
truncate term_node;
truncate users;
truncate node_comment_statistics;

Other Option if you want to drop all the tables at once to restore the new data

drop table comments, content_type_forum, forum, node, node_revisions, term_data, term_hierarchy, term_node, users, node_comment_statistics;