Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
When you see your own comments, on the bottom left hand side should have two clickable options: "Edit" and "Delete". Click "Edit" and up top the your comment, you should see a series of letters and symbols. One of those 'letter' is an "S" with a line across it. Before you click that, highlight the text that you want crossed out and then click the crossed S and then save. That's how you cross out your text.
corystevenbrooks (for Xerneas): Unfortunately, I couldn't find your Cutiefly, it might have gotten sniped? :/ Or if not, is it possible that you made a typo when entering the Pokémon name in the GTS?