Steve,
Replace Text option only works for whole string replacement.
If the srtring you're trying to edit is not supposed to have commas other than trailing, you can use option "Select Text" - "Before Known character" - "," - From Beginning of text.
If commas (other than trailing) may be present within the string, you out of luck as far as the Printer configuration goes, and you'd have to use either SQL script or AutoIndex Job to remove trailing commas