Federal Aviation Regulations specify a minimum altitude of 1,000 feet over congested areas and 500 feet over non-congested areas. The exception to this rule is helicopters and aircraft that are in the process of taking off or landing. To report aircraft that are noncompliant or demonstrating questionable flying practices,  contact the FAA Flight Standards District Office at 1 (800) 847-3808 or https://www.faa.gov/about/office_org/field_offices/fsdo/den/contact/ and click on the email link.