how can i delete categories ?

Use the SQLite console. There are several ways, depending of what actually you want:

I. If you don't need the tag, simply delete it:

  delete from tags where tag='TAGNAME';

II. Or if you want to keep the tag, you can

1:

Set it's importance < 0 (this will hide the tag from the tag list as well, but it still can be used for threads):

  update tags set Importance = -1 where tag = 'TAGNAME';

2: Set its description to NULL (will remove it from the categories, but will be visible in the tag list):

  update tags set Description = NULL where tag = 'TAGNAME';