Then, threaten them.

If they attempt to bill your mother again for anything beyond that, threaten to report them to the state attorney generals office for fraud. Demand that they never again contact her by telephone, because if they do you will report them to the FTC for violations of the Fair Debt Collection Act. Also make mention that if they report this problem, which is their fault because of failure to honor your mother's request to cancel her account, to any credit agency, and it damages her credit, that you will pursue a lawsuit against them for damages far in excess of whatever their bill is.