The preparation of nopales here for cooking (as I would assume is typical through most of Mexico) always involves the removal of the areoles from the pad. Which an expert can do with a sharp knife in about 5 seconds.
You want to look for young pads when you have the choice, because they are the tastiest, and they are also less spiny at that point to start with. I know this because I can do my prickly pear pruning without gloves if I get in there very early in the season. To some extent the youngness of the pad overrides whatever concerns about spines that you might have based on a full grown adult. Our "spineless" nopales are not exactly that but it doesn't matter too much in the end.
Typically the plants in nature (as opposed to varieties selected for cultivation) are really well armed for self-protection, and you wouldn't generally want to mess with those in the first place.
