From SA Mafia Wiki
A Roleblocker is a role that prevents their target from using a night action.
Roleblockers can be of any alignment. Town Roleblockers can block nightkills or neutralize suspcious players, while Mafia Roleblockers can facilitate nightkills by blocking Doctors or protected Investigative roles. A Mafia Roleblocker often cannot claim their role without being revealed as anti-town, either due to roleblock flavor or roleblock choice. Some scum Roleblockers choose to play as if they were town Roleblockers in order to set themselves up for their roleclaim, but this minimizes their usefulness to scumteam.