How do I stop a creditor from billing me after dispute?

My ex-lawyer keeps sending me monthly invoices. I have written several times that I owe nothing and am not paying. She threatened to take it to collections over a year ago but did not. I still receive monthly invoices with interest accruing. Do I mail her every month the same letter that I owe her nothing and am paid in full? How long can she rack up interest and THEN take me to collections? What is the statute of limitations on her quest for my money?