Theyve already admitted it is unenforceable so dont pay them a single penny. They are playing the morality trick on you. Basically " We know we cant make you legally pay for this, so we're going to try and guilt trip you into it".
Pity oft guidance says otherwise. Should they continue harassing, get the harassment letters off. basically theyre ****ed off you called their bluff and know they have never been able to legally collect, but they still want free money off you.