they said they are reluctant to 'sort' them for people any more as they are paying the fines themselves
1/ They are invoices, only a court can fine you (since the Magna Carta), even the police, councils, other statutory bodies can only issue penalties.
2/ Tough, they entered a contract, of their own volition, they put themselves in that position by not doing their due diligence before signing it, lean on them and make a credible threat to move if they don't sort it out (1-2 months of lost membership would cover the invoice). Their bed, they have to lie in it - and if they lie with dogs they will catch the fleas. Enough get annoyed they may bin off these parasites.