OPen up again the template(s) and look for the line:
<div align="center" class="itemPermaLink"><{$catarray.letters}>div><br />
Uncomment them and your done.
If you only want to remove the digits open up: include/function.php and look for the line: (prox. 543)
function PDd_letters()
$alphabet = array ("0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z");
Here you'll see the digits and letters. By removing or uncomment them (preferable) they wont get visible anymore.
Grtz., Shine