تُفْسِدُونَ ← لَا تُفْسِدُوا
You (plural) spread corruption → Do not spread corruption.
The first form is indicative and retains its final nūn. In the second, prohibitive lā governs the jussive, so nūn is removed. The plural wāw continues to represent the subject.
تَفْعَلِينَ ← لَا تَفْعَلِي
You (feminine singular) do → Do not do.
This is another of the five verb forms; nūn deletion marks the jussive. Thus not every prohibitive imperfect has a visibly final sukūn: the jussive marker depends on the verb pattern.