Sunday 5/18/2014: I went into my Planet Fitness in Richmond, California, and said I'd like to cancel my membership. The staff person said I could cancel there in person, but not at that moment, because it was 15 minutes before closing on a Sunday. (This location closes at 7:00 PM on a Sunday.) I asked him whether actually I need to send a certified letter, which I had heard was necessary. The staff person said yes that works too, but the easier way is to put it into the computer right there in the franchise. He said I'm already being billed for May; if I cancel by June 10 them I avoid the June 19 bill.
Next day, Monday 5/19/2014: I successfully cancelled, 9:45 p.m. They have a standard process with a paper form which they then entered into the computer. It was so fast, I asked the guy if we were all done. He turned his computer screen around and showed me the word "Cancelled" on my record.
I wouldn't expect every cancellation experience to go just as smoothly as mine did. You might encounter an irritable staff person, perhaps tied down by a manager who pushes the staff to slow down the cancellation process. My point is, the corporate parent definitely does offer a standard, convenient way to cancel -- paper form plus computer update, right there while you wait. If you experience something different, it might be the individual location departing from the standard corporate process.