One time me and a pal made a pair of sibling Nosferatu blood bankers from Israel. We had each other, and a couple of ghoul PCs played by third parties who were knew OOC. Our strategy was to wait until someone really needed blood bad, and then charge them ridiculous interest rates on it. When they couldn't pay, we'd demand favors from them by pain of "enforcing" the debt, which just meant beating them up and then PKing them. Since there was a group of us, we would rule-of-five them.
A few PCs ended up getting Diablerized, and we would brag about how they essentially consented to it in public, since owing us blood for a long time was not acceptable. However, we could always work something out if you can't pay. It was actually extremely effective, but it stopped working when the ST's stopped enforcing our rolls to track people down. We disbanded without getting banned.