Removing last title character when it is a comma (,)

This removes all trailing commas.
Action: Format value
Field: TITLE
Formatstring: $trimRight(%TITLE%,',')

This removes only one trailing comma, if the last character is a comma.
Action: Format value
Field: TITLE

$if($eql($right(%TITLE%,1),','),$cutRight(%TITLE%,1),%TITLE%)

See also:
Removing " , " at end of Artist field

DD.20110801.1626.CEST