Nostr does not have a delete message and even if it did, people could choose to ignore it, it's a decentralised network.

First rule of signals intelligence is don't send a message over an insecure channel if you don't want it intercepted.

If you don't like it, don't send the message in the first place.

If it is any consolation, there's a good chance it will eventually be deleted when people's disks start to get full of spam. But it's best to assume once sent, forever out there.