Add a text stamp and set it dynamic. Click the configure button and from the script type objects options add a new script and name it FilenameNoExt.
Set the script code as:
function FilenameNoExt() {
var Filename = BatchFile.filename;
return Filename.substring(Filename.lastIndexOf('\\') + 1, Filename.lastIndexOf('.'))
}
You can now type [<FilenameNoExt>] as the stamp text to stamp the PDF filename without the file extension.