Is it permissible to dispose of a usurious loan, from which nothing has yet been spent, after the borrower has come to know of its prohibition?

A Muslim is not permitted to borrow with interest (riba) except in cases of extreme necessity. If your husband has taken out an interest-bearing loan, then if repaying the amount will free him from the interest charges, he should return the money to the bank. However, if early repayment does not exempt him from the interest, then he is not obliged to repay it before its due time, and he should not do so to avoid encouraging riba. He only needs to repent, and you are permitted to benefit from the borrowed money.