There's this song and dance that I see a lot. A user complains that they can't delete their post, high rep users tell them that they need to click the delete button, the user has no idea how to do that, the high rep users get confused how that's even possible when the word "delete" could not be any more clear as to its function, until the post finally gets deleted by some people with mercy and delete votes.
This all happens because the user is unregistered and has no idea being unregistered is why they can't delete their post: there is no delete button. They may vandalize their post, all because they don't realize that they could have done things the right way. (The high rep users who see this happen don't usually realize this either, despite many years of using the site, and may make the situation worse by leaving confusing comments. Plus moderating posts like this isn't fun.)
So give them a delete button. Clicking it that should give them a popup with a button to go register their account (if it's not a question with upvoted answers that no OP can unilaterally delete, in which case that should probably be mentioned instead).
Usually these are posts that the user wants to delete for a good reason, such as after getting a comment that helps them realize their question is off topic or their answer is not an answer. If they could delete outright, it would reduce the number of posts that are vandalized that have to be rolled back, downvoted, then deleted.
Alternatively there's the suggestion Allow unregistered accounts to delete their own posts.
Here's an example of what it looks like for an unregistered user (before the question reached 15 comments, with the one explaining it right getting buried):
See ELU Meta Why can't a user delete their own question that was just asked and has had no activity? for the context surrounding this example. See How should I deal with unregistered users trying to delete their question? for another example of the same issue.