You cannot "unsend" a letter, an SMS, or an e-mail. You can delete messages "for everyone" in many instant messaging apps, and you can do that on Discord, too. The thing here is that they did not do what I could have done were I able to access my account. Perhaps it is covered in their ToS or Privacy Policy.

To the down-voter: could you please elaborate with what you got the problem so we could potentially discuss it?