how to remove category from the posts

That kind of information is usually displayed using the_tags() and the_category() so figure out what Template is displaying those posts and delete the appropriate code.

Review:
Stepping Into Template Tags
Stepping Into Templates
Template Hierarchy

the “wp12” is a category?
if so you can check your permalinks structure or your .htaccess file.
also if you want you can check the WP No Category Base plugin here