Delta and other airlines have shown that their mask policies are not just a suggestion, going so far as to remove passengers from flights if they do not wear a mask.

Passengers who refuse to wear a mask on Delta Airlines will no longer fly with Delta, according to internal communications.

"Although rare, we continue to put passengers who refuse to follow the required face-covering rules on our no-fly list," says Delta CEO Ed Bastian in an internal memo to employees shared with CNN.

That has resulted in about 240 people banned from flying on Delta.

The memo, dated August 27 and seen by several media organizations, was celebrating the opening of Delta’s new Salt Lake City hub, and included an update on the current state of the airline.

Since implementing a mask policy in June, the number of passengers on the no-fly list has more than doubled. Bastian told CNN on August 7, "We've had well over 100 people that have refused to keep their mask on during the flight."

That reportedly includes a former Navy SEAL who claims to have killed Osama bin Laden. Robert O’Neill tweeted about being banned by Delta Air Lines after removing his face mask during a flight.

There are also recent stories about flights returning to gates when passengers remove their mask before take-off. Some passengers are removed if they refuse to put the mask back on.

Delta requires passengers to wear a mask starting at check-in, and has asked passengers who cannot wear a mask because of health conditions to consider staying home.