I've never had any luck "adjusting" the brake light switch. They're designed to be a "one time user" self-adhusting unit. When new, the plunger is all the way out and after installation the brake pedal contacts and pushes the plunger in. It will go in further, but where the pedal stops is where it needs to be.
My guess is that in the process of replacing all the master cylinder or another item, the mechanic inadvertantly pushed the plunger in a bit further than it was. If you can "pull it out", more power to you. I've just never had one that would do it. I have taped something on the pedal arm to take up the space. There's really no pressure on it, so it doesn't take much to keep it there. I wouldn't use anything much thicker than the space you need to take up.