Recently, I came across a question that had been vandalized by the OP. I left a canned "Don't vandalize posts" comment I use for these situations that contains the text:
... If you want to know more about deleting a post please see: How does deleting work?
The OP responded that they can't actually delete the question, which is true, since they are unregistered. As mentioned in the FAQ on deletion:
Unregistered users (users who haven't registered their account - that is, associated a Google, Facebook, or Stack Exchange account with it) can't delete any of their own posts.
There have been requests for this feature, e.g. Allow unregistered accounts to delete their own posts, but the feature is not currently implemented.
How should I deal with such a question, which doesn't have any answers, and which the OP would be able to delete if they had only registered their account?
Should I ask the OP to register their account? I already suggested they do that, but that didn't work (for reasons I didn't inquire into), and may not be what the OP wants to do anyway.
Should I flag the question for deletion on the OP's behalf? The OP can't do this, since they have less than 15 reputation, and as far as I can tell that requirement is not waived for flagging one's own posts. I'm not sure if this is an appropriate use of a moderator's time though.
Should I tell the OP not to worry about it, now that they've done everything they can by leaving a comment? That does mean the question might just hang around indefinitely, and that's probably not a good result. Also, the OP may try to delete the post again by vandalizing it, which creates more work for other users. This will raise a moderator flag, who is then likely to delete the question, but this is probably not a reliable system.
Something else ...?