On the Mac platform you can only cancel a test/area render if you've used the camera icon tool, NOT the keyboard shortcut. In conversations with Eovia, canceling a keyboard shortcut apparently requires a substantial rewrite in code - time that they indicated they'd rather spend on new features. (I tend to agree) But like you, there are times that I've forgotten I have light effects on, do an area render, and then scream, "What have I done?!" and either go eat lunch or force quit the program.