They are required by law to follow the Fair Debt Collections Practices Act. From your facts, it doesn't seem like they have violated that. They are allowed to threaten you with a civil action and tell you about the methods they can pursue to collect. If you owe the debt, settle it. Otherwise, they will probably sue you, win, get a judgment, and then collect your bank accounts and wages, and put a lien on your real property.
They've done nothing illegal. You have no recourse. But you should try to avoid suit and a judgment. Pay your debt if you owe it.
This is not legal advice nor does it create an attorney-client relationship. This is for education and informational purposes only. It is always recommended that you contact an attorney with any concerns as each individual case is unique.
The Fair Debt Collections Act offers some protection. From what you have provided, it does not appear to have been violated. If you do owe the money, they can proceed and garnishment and other collection activities are possible. If you don't owe it, ask them for the documents they are relying on, and if they are not yours, challenge them.
The above answer is in general terms, and is not meant as legal advice. In order to fully answer the question, more information would be needed.