Lot's of interesting ideas and opinions on this matter.
I haven't been here long enough to see the problem on the long term, and for the short term I've been lucky that everyone I've dealt with in trades has been honest and solid so far.
I think banning things like chat, client, etc, only work if it's a value to the user in the first place and even things like a wall of shame aren't very effective if the user has no real pride for their reputation.
I do think Zab might be on to something with the tax idea, although I wouldn't do it with the client because again, it only matters if the user uses it. Rather I would take the same approach the taxman takes on defaults and garnish 50% of all points earned from said user with a 10% additional fee going back to PO for the hassle until everything is payed off.
Earning points is the one thing every user wants and has to do on here to get anywhere, therefore when attempting to punish, it's the most effective place to crack the whip.
The decision to put a user in default like that is a tough one for a person to always make, so what I would recommend is including a payback date in all trades that the borrowing user understand will trigger this automatically.
Those serious about being productive with their poker earnings on here should find that a date to take serious if they want to continue to do so.
Tonis