Some school labs had left access to Hypercard available through that program, so you could just pull up Hypercard and make a new stack that would tell the program to quit.
The other way to get around it -- or around many other misbehaving programs in the cooperative multitasking system -- was to bring up the programmer's interrupt like the author describes, and enter "SM 0 A9F4", followed by "G 0". This would set memory location 0 to the _exitToShell function in the OS ROM and then resume execution from there, which immediately terminated whatever was in the foreground.
The rest of the system would often be a little unstable after that though, so you only had a few moves left before a restart would be needed.
To youths with curious mindsets, an anti-authoritarian streak, and seemingly limitless amounts of free time, little restrictions like these only improved our skills. Systems with challenging but imperfect security are a great way to foster new young talent.