Apply the same rule to different verbs.
ProhibitionCommandMeaning
لَا تَنْصُرْاُنْصُرْhelp
لَا تَضْرِبْاِضْرِبْhit
لَا تَسْمَعْاِسْمَعْlisten
The command changes its beginning, while the prohibition keeps the Mudhaari' prefix and adds prohibitive Laa.