Unfortunately, putting them straight into the freezer is not humane at all. The extreme drop in temperature does not cause them to fall asleep, it in fact causes little crystals to form in the cells of the body, causing something akin to frostbite. By this method, they die from extreme cold or suffocation (if they're put into plastic bags first).
The correct method of doing this is putting the cane toad into a fridge beforehand (and this only works effectively for amphibians), for as long as possible (overnight is preferable), and THEN put the toad into the freezer. The decrease in temperature is not as sharp as simply putting the toad into the freezer, and thus it gives the toad a chance to "fall asleep".
Please don't think any of you are being humane by putting ANY animal straight into the freezer