Should ask the admin to do IP mapping for the suspected accounts of him. ( as you all claims the kawasaki and marlboro user id )

then maps them with the current user.
Those current user IPs with same ips as the suspected conman, should then be shortlisted into those that does trades in here. Especially newly registered username .

Then trade enforcer should closely monitors these people and see if there's any identical trading styles to the conman to prevent concase from happening again.

those who are unlucky to have same IPs as the conman, should be asked to verify themselves to the administrator.

Just a suggestion.