Worst case, your "debt" can be handed to a collection agency which will be a pain for you, even if you are not owing them anything. From a practical point of view, I would first demand that they acknowledge the mistake in writing, and the next time ask a lawyer to write a letter for you and charge them for it.
Comments: Why "Ask a lawyer to write a letter and charge for it". A lawyer will know what to put in the letter so they have to pay him his fee, because they are asking for money that they have no right to. Clearly someone doesn't care about the inconvenience of it all to you. Once they have to pay $100 for this letter, they will care and make sure the inconvenience to you goes away. Their boss will ask someone "What is going on here? Why are we paying $100?" and someone has to say "because we asked a customer repeatedly for $25 even after telling him, including in writing, that he doesn't owe us anything."