The current workaround here is to ask them to flag, in practice posting a topic and then changing mind about it is quite rare.
If we wanted to be ultra fancy here I guess we could allow end users to tombstone and close the topic if it has no replies (waiting 24 hours till actual deletion). In practice we find it ultra rare for users to change mind about posting a topic. If they do they usually follow up with, can you please close this it is a duplicate of: <link to topic> (either in a flag or followup reply)