If a broker closed your account unfairly, these are the legitimate places to file. Every link and number below is an official source.
Online: finra.org/investors/need-help/file-a-complaint
Phone: (301) 590-6500
Mail: FINRA Investor Complaint Program, 9509 Key West Avenue, Rockville, MD 20850-3329
Robinhood Financial is a FINRA member (CRD #165998), so your complaint is forwarded to the firm and it is required to respond to FINRA.
Online: sec.gov/tcr and sec.gov investor complaint
Investor Assistance: (800) 732-0330
Online: consumerfinance.gov/complaint
For Washington State residents stuck with a federal agency (constituent casework):
Sen. Patty Murray: murray.senate.gov/help-with-a-federal-agency
Sen. Maria Cantwell: cantwell.senate.gov/contact
Rep. Rick Larsen (WA-02): larsen.house.gov/contact
Rep. Suzan DelBene (WA-01): delbene.house.gov/contact