I have never used MSSQL but if it really supports SQL
DROP TABLE phpbb_auth_access;
DROP TABLE phpbb_banlist;
DROP TABLE phpbb_categories;
DROP TABLE phpbb_config;
DROP TABLE phpbb_disallow;
DROP TABLE phpbb_forum_prune;
DROP TABLE phpbb_forums;
DROP TABLE phpbb_groups;
DROP TABLE phpbb_posts;
DROP TABLE phpbb_posts_text;
DROP TABLE phpbb_privmsgs;
DROP TABLE phpbb_privmsgs_text;
DROP TABLE phpbb_ranks;
DROP TABLE phpbb_search_results;
DROP TABLE phpbb_search_wordlist;
DROP TABLE phpbb_search_wordmatch;
DROP TABLE phpbb_sessions;
DROP TABLE phpbb_smilies;
DROP TABLE phpbb_themes;
DROP TABLE phpbb_themes_name;
DROP TABLE phpbb_topics;
DROP TABLE phpbb_topics_watch;
DROP TABLE phpbb_user_group;
DROP TABLE phpbb_users;
DROP TABLE phpbb_vote_desc;
DROP TABLE phpbb_vote_results;
DROP TABLE phpbb_vote_voters:
DROP TABLE phpbb_words;