From SA Mafia Wiki
A Roleblock is an active night action that causes the target's night action to fail. Roleblocking is useful for both the Town and Mafia, as the Town can use it to keep harmful night actions such as nightkills from happening, while the Mafia can shut down dangerous town power roles without having to kill them.
Roleblock flavor often reveals something about the roleblocker and usually stays constant from night to night.
Roleblocking priority varies from moderator to moderator. Some roles, such as Bus Driver, Hider, Lightning Rod, or Nexus, can take precedence over roleblocking. Generally, roleblockers are unable to roleblock hiding roles (as they are unable to find them) and Lightning Rods/Nexuses (as those roles are typically one use only and tend to take priority over a redirected roleblock).