There is no OOB way to solve this for a variety of reasons
- The Lead Conversion can execute triggers on the converted-to-objects. These triggers could create new objects or even do callouts to external systems
- The Lead Conversion can also fire workflows and process builder flows that in turn send emails, do outbound messaging, create related objects, use Apex invocable methods, etc.
As the scope of related, downstream changes is hence unbounded, there is no way SFDC can roll this back.
Consider yourself lucky that you realized you made the mistake and clean it up manually by recreating the
Lead from the Account+Contact data and deleting the converted-to
Contact. If emails were sent out, you may ned to manually construct an email apologizing for the errant user action