I've heard returns are a big headache with clickbank. Do they protect vendors by filtering out fake customers who have no intention of paying (the ones that constantly return ebooks, etc.).
Yes, if there is a customer that constantly refunds the products he buys, they refuse any further transactions from that customer
Yes I think they have measures in place. I remember reading a member on dp complaining why they were banned from using clickbank because they made so many refunds.