Just a suggestion:
On the left side of each post we make is our Join Date, how many Posts we've made, and Money Won. Would it help/be possible to have screen-names registered on sites you plan to use for trades/loans listed as well, and by having them there means they have been verified by PO Admins? Further, any time a player has a history of ANY kind of impropriety with the funds of others, simply have a link/logo there as well. A kind of "badge of dishonor," if you will.
So with that info attached to their name, ppl with multiple accounts would also have to have multiple accounts on the poker sites they use as well (sure, it happens) or risk exposing the fact that they are trying to use another name here. Additionally, people considering entering any transactions with them could instantly link to any threads covering any past issues. There could be "badges" for not re-paying loans, for past multi-account activity, for not returning stakes, and perhaps some way to distinguish if the issue was resolved.
While I agree that the type of person who is careless with others funds is more likely to repeat such behavior, there should be ways to clear your name, so to speak, but not to erase your history altogether. Some kids learn respect, others just learn new ways to disrespect.