How do you challenge a collection agency from illegal collection?

If you believe a collection agency has done something illegal you you should contact an attorney. In some rare cases you can bring action through your State Attorney Generals Office, Federal Trade Commission, or the Consumer Financial Protection Bureau. If those agencies don't have a remedy than your only recourse will be to sue in either State or Federal Court.

Most laws regulating collection activity are Federal Laws (FDCPA, TCPA, etc) however many states have additional laws and thus some state courts would also have jurisdiction.

If you are simply disputing the validity of a given debt than the following actions should be followed. If you feel the debt is not owed and it is reporting to your credit, dispute it with the 3 major credit bureaus ( Equifax, TransUnion, and Experian). Then contact the collection agency via certified mail to dispute it as well and ask for proof that the debt is valid.