Force spaces instead of tabs

[Martin]

I need to have emacs indent C++ code using only spaces, and no tab
characters.

I edited the options in the Programming/Languages/C group so that:
C Basic Offset is 3
C Tab Always Indent is "always indents"
C Insert Tab Function is "insert-tab"
C Syntactic Indentation is non-nil
C Offsets Alist: substatement-open is 0

In many cases, the indentation is correct, but when I have several
levels of sub-statements, tab characters are inserted anyway. See the
example below. What other options control this behaviour?

void B::e()
{
   if ()
   {
      ;
      if ()
      {
         ; // tab character at beginning of this line!
      }
      ;
   }
}