Removing automatic comment leaders
From Vim Tips Wiki
created 2001 · complexity basic · author Benji Fisher · version 6.0
If you include the "r" or "o" flag in the 'formatoptions' option (:help 'fo', :help fo-table) then the comment leader is inserted automatically when you start a new line in a comment. For example, in TeX the "%" character is the comment leader, and you might type
% This is a tex file. % The comment leaders on all lines but the first were generated automatically. % This is the last line of the comment, but Vim will insert the comment leader on the next line. %
You can get rid of the comment leader (along with anything you may already have typed on the line) without affecting the indent, if any, by typing
<C-U> while in Insert mode (in case you do this by accident, there are ways to undo it).
Related point: If you want to adjust the indent while in insert mode, you can use
<C-D> (to decrease the indent) or
<C-T> (to increase it). See :help 30.4.