Stop paying! Have you spoken to your neighbours? What did the note say? If anything, they would be the best people to chat to about suspicious characters.
Also, I recently went to a Town Council meeting even though my ah longs haven't done anything to me yet (touch wood). I was told there are around 50k debtors in Singapore and thousands of unlicensed money lenders. Statistically, that's about 50 debtors to one UML, so that should give you an idea of the debtor turnover rate. I gathered that the possibility of them terrorizing a debtor has a lot to do with amount owed, and the debtor's likelihood of borrowing again. Because it means that the debtor has cash flow/gambling problem. Otherwise, their attacks appear to be arbitrary in nature. So IN THEORY, without doing anything illegal, people can actually borrow a reasonable sum of money (between 2-4k) with absolutely no recourse to the UML if it is defaulted. And it won't make economical sense for the UML to spend money to harass you if you defaulted on a loan of that amount. IN THEORY, if more people borrowed that amount of money from UMLs and defaulted, we could actually take down that arm of their network. Because life is a Die Hard franchise...
Having said that, I just installed a camera outside my house. You want contact details?
P.S. Obviously, I'm not encouraging people to involve themselves with UML. I like to think people who come to this website are capable of high level thinking even if we were silly enough to borrow from loansharks.