My understand is that when you purchase a gift voucher you become a creditor to HMV. Creditors should always be paid from largest value of money owed to the smallest. By temporarily refusing gift cards HMV are simply restoring the correct creditor order, so that the creditor with most to lose doesn't do so at the expense of other creditors
You can sue them, but try seeing how much money you'll get....
They know it's illegal, but also know they can get away with it.
This happens every time a company goes under. Instead of blaming them, blame the prat who bought you a gift voucher instead of just giving you the cash!