Just because they refused to take your payment does not mean the debt is paid in full. In fact, it usually means the opposite, when a payment is refused that means they are refusing to take your payment or provide you a service. I am sure there is more going on here than what you said, but this seems a bit odd that they would not accept your payments (I am guessing you are fairly delinquent). Do they give you a reason for refusal? Usually a company will at least tell you why they are refusing to accept your check or credit card.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Your posting did not explain why the company refused your payment attempts. Perhaps they are not set up for credit cards or you need to go there to make the payment in person with the card. Perhaps they can accept PayPal, if you cannot go to the facility during business hours or you can mail them payment by check.
That said, there are a couple of California rules of law that you should know. Cal. Civil Code Sections 1485 to 1515 should be read carefully, so you understand them. In particular, Section 1485 states that an offer of performance extinguishes the obligation, but Section 1486 says that partial performance is of no effect. Section 1512 states that if the creditor prevents full performance, the debtor (you) is entitled to the benefits of full performance.