Is it permissible for me to repay my father's usurious loan again, knowing that my previous repayment encouraged him to borrow repeatedly, and thus I became a "support" for him in that?

The child is not obliged to pay off his father's debt, even if he is able. However, if he knows that paying off the debt will lead to repeated usurious borrowing, then he is not permitted to pay it off. Nevertheless, the child must spend on his needy father if he is able. And if the father borrows to meet his essential expenses, then the son must enrich him from borrowing by spending on him with what is reasonable.