Tell  me what you believe your legal options are.  For a start, the amounts owed to most people would likely fall into the realm of small claims and small claims is an area which often requires the person making the claim to be present at the hearing.  It always requires you to prove your claim and often requires you to demonstrate that the person you're seeking judgement against has been given an opportunity to pay and didn't - ie, that payment is "overdue".  

If you're going to bypass small claims and try for individual lawsuits, then you're running into a real problem both with expense and with the court examining whether or not the activity in which people were engaging with GLBSE was legal.  As you can't file a lawsuit anonymously, you are tying yourself to an illegal activity in a legal forum in order to pursue an amount which is probably less than the fines you'd be facing for participating in that activity.  You'd be a fool to try to represent yourself in any such action, obviously, and international lawsuits are not cheap - you'd be looking at paying lawyers in at least two locations, and your cause of action is what?  That you lost money as a result of Nefario closing down an illegal enterprise?  Good luck with that argument.

Edit.  Just checked UK Civil Rules.  Here's what you're required to do prior to starting court action - including small claims action - against someone in England, Wales and Northern Island.  Annex A is especially important.

http://www.justice.gov.uk/courts/procedure-rules/civil/rules/pd_pre-action_conduct