Can I delete a message that the receiver won't see it anymore

Answer 1

No, you can't delete messages.
If you use the "Delete Conversation" option, then the conversation will be deleted only from your list, another person will still see all the messages.

Answer 2

A private message? If you sent it, they can see it.

If you sent a message that you wish that you didn't all I can advise you to do is to think before hitting that "send" button next time.

If it was a comment, you can edit them.