I usually try to get to an agreement with any borrowers about what happens to their account if they default. For example, let's say that a user with an account worth .3 BTC wishes to borrow .085 BTC for 10 days, with a repayment of .1 BTC. I'll say that in the event of a total default, the user has the option to sell the account back to me for .05 BTC. If they get negative trust the account becomes worthless anyway, so the extra few bucks is better than nothing.
In theory, that works beautifully. In practice, you end up spending two months for the buyer to "pay back next week" and for bitcoin to go down 40% before finally getting your money back.
In other words, it works in preventing pure defaults, as long as the account that is borrowing from you is worth more than the loan amount. If the account isn't worth more than the loan amount you're making a mistake in loaning to them in the first place.
In your particular case, I'd recommend inquiring to these users about their accounts. I bet leevalle would give you his account if you took down his ID. Anshar may be willing to sell his account to you for a nominal amount if the sig campaign proceeds thing doesn't work out. There is always a solution - if someone permanently defaults on a loan and isn't senile, dead, or in prison, then you're doing something wrong.
I don't think the account could possibly be transferred to you. The passwords should be encrypted therefore you can never find out the original passwords of the scammers.
Pretty sure admins can change someone's password without knowing their original password.