Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I presume you have and are asking about a Canon EOS 40D digital camera (and 'ImageClass' is a Canon laser-type printer line... so, I'm guessing to some extent, I admit.)
Try setting the Auto-Power-Off setting shown here:
Turn the dial to select Auto Power Off, press SET, turn the dial to select the time you want, then press SET.
If you set the auto-power-off, wait that amount of time, then the camera *will* power Off. Period.
That may be advice for a different model of printer. The holding down of the STOP/CANCEL button would be a standard sort of hold-button-down-while-turning-On method. It might work with this model though.
As for turning it Off before unplugging it - if you unplug it sharply and suddenly, it doesn't need to be turned Off first, and in fact having it On when you unplug it can help drain electric impulses and stored charges from the circuitry.
I'd advise waiting 20 minutes, not 70 seconds, though. Try it.