UNITED STATES - FEBRUARY 11: Guests ride an escalator in the background of a Marriott logo on a window outside a Marriott hotel in New York, U.S., on Wednesday, Feb. 11, 2009. Marriott International Inc., the biggest U.S. hotel chain, fell as much as 9.3 percent in early trading after reporting a fourth-quarter loss and forecasting more weakness for the travel industry this year. (Photo by Daniel Acker/Bloomberg via Getty Images)

Hotels, beware: Blocking guests' access to Wi-Fi in order to force them to pay for your own costly Internet service is not a very nice thing to do. And, it turns out, it's not legal either.

The Federal Communications Commission announced Friday that it was fining Marriott International $600,000 for unlawfully blocking customers' Wi-Fi access in a conference room at the Gaylord Opryland Hotel in Nashville, which is operated by the company.

According to a statement from the FCC, Marriott workers were using "Wi-Fi blocking technology" called jammers to prevent people at the hotel from using the Internet via their own personal Wi-Fi networks they established with smartphones or mobile Wi-Fi devices. Meanwhile, the hotel was charging $250 to $1,000 per device to connect to its wireless network in its conference facility.

"It is unacceptable for any hotel to intentionally disable personal hotspots while also charging customers and small businesses high fees to use the hotel's own Wi-Fi network," said FCC Enforcement Bureau Chief Travis LeBlanc in a statement. "This practice puts customers in the untenable position of either paying twice for the same service or forgoing Internet access altogether."

How does a hotel block Wi-Fi in the first place? The FCC defines Wi-Fi jammers as "illegal radio frequency transmitters that are designed to block, jam, or otherwise interfere with authorized radio communications." Marriott was using these jammers to block people's personal Wi-Fi hotspots.

Though the FCC is telling Marriott to stop the Wi-Fi blocking, the hotel chain maintains that what it was doing wasn't illegal.