Fortunately, as of late Monday, Apple reportedly took FaceTime's group call abilities offline while they worked out a fix. However, if you want to be extra certain that no one can listen (or see) in on you when you're not aware of it, here's how to disable FaceTime on your devices for the time being.
How to Disable FaceTime on Your Devices
On an iPhone or iPad
First you'll need to open Settings then scroll down to FaceTime, where you simply need to switch the green button to disable FaceTime.
On a Mac
Find the FaceTime app and launch it (if it's not on your dock already, you can easily find it in Finder under Applications). Then, tap the FaceTime menu up top and select the option "Turn FaceTime Off." Voila, that's it. If you want to be extra careful, you can also easily sign out of FaceTime altogether on your computer. To do this, open your FaceTime app again and click the FaceTime menu up top, then select "Preferences." From there, you just click sign out.
Hopefully, Apple will be able to address the concerns and issues sooner rather than later, and add some protections to ensure this sort of glitch isn't as easily exploited going forward. Then again, we don't blame you if you retire FaceTime from your favorite apps list for a while.