Can I unsend an iMessage after it has been delivered?

If they have received and read it, no. If they haven't read the message you have up to two minutes to unsend a message. Anything longer than that, you can't unsend it. See this --> How to unsend messages on your iPhone - Apple Support

“delete converstions from recipant: I’d like to delete my iMessage from receiving party. Is that possible?“

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Oops!

If no time remains to undo this, inform the recipient that it was mishap, and you copy and pasted the incorrect info.