you can do it by two ways:
in your mysql you can TRUNCATE TABLE qa_posts;
you also can DELETE * TABLE qa_posts WHERE postid > 0;
The difference between them is (1) Truncate is more quick than Delete; and (2) Truncate will reset the primary autoincrement key (postid) to 1, but delete will preserve the postid key.
But you also need to clear out all other postid related tables expect below. For example:
Alternatively, you can user export above tables to your pc, then delete whole database, then re-install and import above tables.