That wheel is the viewfinder diopter adjustment. It changes the optical focus of the viewfinder so photographers with minor vision differences can see the finder image and display clearly, with or without glasses.

It only affects what you see through the optical viewfinder—it does not change autofocus, Live View, or the sharpness of the actual photo. That’s why your images were still fine.

To set it correctly, look at the viewfinder’s numbers and focus indicators and turn the wheel until those appear sharp. You can also use Live View to confirm the lens is focused, then adjust the diopter until the viewfinder looks sharp too.

There usually isn’t a way to disable or lock it, since it’s a simple mechanical adjustment. If you keep bumping it, a small piece of tape over the wheel can help. There is typically a default or “zero” position marked on the camera/body alignment marks.