Tell me how to cross out the message (list of already activated keys in description)?

Are you talking about this?

~~ marks on both sides of the text.

I do not understand what you are asking. If you are talking about formatting text on steamgifts, there is a guide here: https://www.steamgifts.com/about/comment-formatting

If you mean 'giveaways won' message, you need to press received after you acrivate the key on your own steam account.

Tell me how I can be so vague.