Delete that Section Tag Tail

1. From the menu select "Settings" -> "View Tags" -> "Section Tags"
   
2. "Duplicate"
   
3. Name the new section tag accordingly.
4. Change "Section Tail" to "none". Close all dialogues.
   
5. Select an existing section tag.
6. Modify properties, "Edit/New"
7. Change "Section Tag" to the style created in the previous steps.
   

See also: http://revitrants.blogspot.com/2006/04/making-elevation-tags-look-like.html