The CFPB says anyone sued over an unpaid debt should respond, personally or through a lawyer, by the date in the court papers. Responding does not mean agreeing that you owe the debt; it makes the collector prove the debt is valid, and you may be able to negotiate a settlement.
If you do not respond, the court can enter a default judgment for the amount claimed plus lawful collection costs, interest and attorney fees. Refusing to accept delivery of the lawsuit is unlikely to work, since a court may treat that as ignoring a properly served suit. A judgment is a court order, hard to change afterward, and you may lose the chance to dispute the debt. Depending on your state, the creditor may be able to garnish wages, place a lien on property or move to freeze bank funds.
The agency suggests consulting an attorney, ideally one experienced with the Fair Debt Collection Practices Act; some offer free or reduced-fee services, and legal aid clinics may help if you meet their criteria. Servicemembers can consult their legal assistance office.