Select the rows you wish to comment out. Press Ctrl+H to bring up the Find and Replace tool. Click the Regex button (in the square in the image below). In the Find field, type "\n" which is the new line character, and in the Replace field, type "\n#" which will add a # after the new line. Then replace all.
How to Comment Multiple Lines in MET/CAL Editor
Smart way! If there is a keyboard shortcut will be much better.