oil company trying to sue me for the debt i did not have with them

In 2009 my case was transfered to a different oil company due to the illegal issues of the original company. The original company claimed that they delivered oil to my house. the issue was never resolved before the case transfer. The new oil company is trying to charge the amount from me since 2009. i have explained them the situation many times, they still persist with the claim. Now they claiming the "original" amount and the late fees, almost $3,000. What can i do to prevent this case from escalating?