How to treat the people who borrow money but never pay it back?

Actually if you always help out somebody with cash, all that you do will be to develop their negative habits. So instead of better managing their expenses, they will know they can always borrow their way out of their financial conundrum. In this case, what I would suggest would be to better give advice and help that person out with her financial problems, instead of offering interest-free money. Because going in debt only postpones the problem, but doesn't solve it. Nonetheless, if it were possible for her to identify the main points where she loses money and she sorted those issues out, maybe she could come on a stable financial level.

I don't think it's about offending A, because it's B's right to do that. Maybe try to talk to A nicely and say that she needs to use the money?

A is doing that to B , because B doesn't complain . But once B realizes that A will keep doing it if he doesn't voice out hid dislike about it , A will always do it . B has to let A know about what he feels about what A is doing . He has to stop A .

I blacklist them on my list who I'll allow to borrow to me haha

It is better not to borrow money to A.