Well, as I alluded to earlier we can't specifically tell you who to block, as if the mafia know that one of their members is being blocked they can have someone else do the killing that night (depending on the mod). It is a decision you will have to make yourself and keep secret until the right time. You just have to use your best judgement for who you think is scummy. Once you have more experiance and have played with the other players in the game to get a general feel for how they act this get's a little bit easier, but is rarely easy.
The ultimate goal with a town roleblocker is to block the mafia nightkill. Not only does that keep a townie alive for another night, but if you can positively conclude that said roleblock is what stopped the kill (easier said than done unless the mod is giving hints) then you can use that to direct your lynching efforts.
Hitting other scum member, while not as ideal, is still useful of course. In addition to disrupting whatever secondary powers they may posses, it can be useful for revealing fakeclaims if they scum don't realize they were blocked. The key there is to not claim who you blocked until they fake an action on the night you blocked them. Of course, this creates the situation where you have to find the balance between the reward waiting on information until the right time against the risk of taking it to your grave should you die before that times comes.