I had a reservation (prepaid with no cancellation/modification) recently for the Four Points at the airport in Boston.

Unfortunately, Delta canceled my flight as I was checking into the airport. I was abroad and couldn't use my phone to call SPG. So, I sent a direct-message to SPG on Twitter as well as an email.

It took about an hour but I received messages through both channels that my reservation was cancelled without charge or penalty. SPG never questioned it or asked for verification, so I was content.

For what it's worth, I'm a SPG platinum but only by virtue of my Marriott status. I stay at a SPG hotel maybe a half-dozen times a year.