What is the ruling on warning against a specific drug by name, and could that lead some to seek it out, and how should a person committing a sin be advised gently?

There is no harm in warning against a specific drug by name. Your advice to the addicted person should be given privately, gently, and kindly, explaining to him the seriousness of sins and how they lead to the loss of blessings, the onset of calamities, and the ruin of the servant. Emphasize that you only wish him well and fear for him God's wrath and punishment.