There should be a button with that appearance in the lower right-hand corner of your comments, which brings up a tooltip labeled "retract" when you mouse over it. Using it will strikethrough the entire text of your post, which 'round these parts is shorthand for "I, the author, no longer endorse this comment". Using it for a second time will delete your post, unless there are responses to it.

There isn't any way to strikethrough portions of a post with LW's markup. Or at least I wasn't able to find one the last time I looked into this. The usual Markdown syntax is disabled here, probably to reserve the look for the retract option.