When you ban dissent, anyone protesting in a security buildings area of effect will be detained indefinitely. When enough colonists are detained the rest of the population gets fearful and becomes less likely to protest. However, if you abuse the system then the people will lose their fear and riot. During a riot, all your buildings will be damaged looted, and many colonists will try to escape your colony. To contain a riot you need riot police, the more of them you have the better because more riot police means less damage and looting will occur.