There usually isn’t a built-in lock for the diopter control, but there are a few practical fixes.

The simplest is to cover or brace it with a small piece of gaffer’s tape or other removable tape. That helps stop accidental movement and can be removed cleanly later. Some people also add a visible mark, such as a tiny dot of nail polish, so it’s easy to reset if it does move.

If your camera accepts an eyecup and you don’t already use one, adding one can help shield the diopter control from being bumped.

A more permanent option on some Canon bodies is to remove the diopter wheel entirely: set it where you want it, hold it in place, remove the center screw with the correct screwdriver, lift off the wheel, and replace the screw. That prevents accidental changes, but you’ll lose quick access to readjust it.

So yes, you can either protect it, mark it, or in some cases remove the wheel; otherwise, checking it before shooting is the fallback habit.