In my opinion nitpick is more natural without an object. You should change this and this. Stop nitpicking. Don’t nitpick. His nitpicking is getting on my nerves. You can also use “about” Stop nitpicking about the minor differences. It’s a 90% overlap with your proposal.

Hello, Igor. You wouldn't use 'at' or any other preposition. You could say: - Stop nitpicking me. - Stop nitpicking my work. I hope this answer helps. If you are interested in increasing your English fluency, I am a professional English teacher on Italki. I would be happy to work with you. Have a good day!

.