"Narcos" producers are warning Pablo Escobar's brother to back the hell off trying to cash in on the popular Netflix show's name and fame ... according to docs.

Lawyers for "Narcos" fired off a cease and desist letter to Roberto Escobar's company telling him he's got no business filing trademark applications for "Narcos" and "Cartel Wars" because both terms are very clearly already attached to the show. "Narcos: Cartel Wars" is a popular video game.

In the letter, obtained by TMZ, the producers say Pablo's older bro is making some outlandish claims in his applications. For instance, they say he claimed to use "NARCOS" in connection with a website in 1986 -- but they're calling BS because the internet barely existed back then.

A rep for Escobar's company tells us their attorney is in discussions with "Narcos" to settle.

Discussions. What could go wrong?