Because the behaviors are intrinsically motivating for sadists, comment moderators will likely have a difficult time curbing trolling with punishments (e.g., banning users)

Ultimately, the allure of trolling may be too strong for sadists, who presumably have limited opportunities to express their sadistic interests in a socially-desirable manner.

http://www.slate.com/articles/health_and_science/climate_desk/2014/02/internet_troll_personality_study_machiavellianism_narcissism_psychopathy.html