Unfortunately, with people like these, you gotta be blunt. You have to say “excuse me, you know I’m on bar, right?”
I worked with a partner like that. One day she was on bar and I was on cold bar, and she kept turning around and making the drinks on my station.
I had enough of it and I said this exact same thing to her and her excuse was that she was helping.
Uhh.. no. We were having a slow but steady flow that morning, there was no need for her to get out of her position other than she liked to be an annoying smartass.

My advice is always, you only gotta make your point once, really clearly and they'll get it
I.e., get out of my way im on bar and im never going to get better if you are in my way

We have a few baristas who do this too, one time though, barista #1 just came back from like a week long vacation, she was on DTO, and I was on bar. For some reason while I was finishing some frappes she grabbed a sticker and a cup and said "oh wow I dont remember how to make this!" I was so tired of her shit that day I said "well are you on bar?..no? Okay so don't worry about it." I definitely have an attitude towards her..but she does the most annoying things.

This happened to me when I was bar and a borrowed was on front till (prior to pandemic). Was nowhere to be found when I actually needed help and they were CS lmao