14 CFR §250.7

1. (a)Boarded passengers. A covered air carrier may not deny a revenue passenger traveling on a confirmed reservation permission to board, or involuntarily remove that passenger from the aircraft, once a revenue passenger has:
   1. (1)Checked in for the flight prior to the check-in deadline; and
   2. (2)Had their ticket or boarding pass collected or electronically scanned and accepted by the gate agent.
2. (b)Limitations. The prohibition pursuant to paragraph (a) of this section shall not apply when:
   1. (1)There is a safety, security, or health risk with respect to that revenue passenger or there is a safety or security issue requiring removal of a revenue passenger; or
   2. (2)The revenue passenger is engaging in behavior that is obscene, disruptive, or otherwise unlawful.
3. (c)Rule of construction. Nothing in this section may be construed to limit or otherwise affect the responsibility or authority of a pilot in command of an aircraft under 14 CFR 121.533, or limit any penalty under section 46504 of title 49, United States Code.