No court would entertain a claim where legal tender in satisfaction of a financial obligation has been refused.
You could play an interesting game of chicken with them: when your next payment is due, present yourself in person with the exact amount in legal tender (i.e. don't mess about with heaps of coins, and don't seek change); if they refuse, then take the position that the instalment for the month is no longer due to them, as they refused your payment. It might help to have a witness.
A few months of this could save you thousands.