test.cs

private void Mm_FieldMerged(object sender, MailMerge.FieldMergedEventArgs e) {

  var keyword = "%REMOVELAST%:";

  // return if field is outside block
  if (e.MergeBlockName == "" || e.MailMergeFieldAdapter.TypeName != "MERGEFIELD")
    return;

  // convert to the MergeField
  MergeField field = ((MergeField)e.MailMergeFieldAdapter);

  // if "text after" contains keyword
  if (field.TextAfter.StartsWith(keyword)) {
    if (bLastBlockRow == true) { // remove "text after" when last row
      field.TextAfter = "";
    }
    else { // else keep "text after" string
      field.TextAfter = field.TextAfter.Substring(
        keyword.Length, 
        field.TextAfter.Length - keyword.Length);
    }
  }

}