You would have to us an if statement in possibly 3 templates. 
In board index > boardindextemplate if you don't want them to know they exist. 
In forum view  > forumindextemplate if you don't want them to see the topic listings 
In topic view  > topicviewtemplate if you don't want them to see the actual topic posts 
  
use something like this if 
<if test="enoughPoints :|:$this->memberData['pp_reputation_points'] > 49"> 
at the top of topicviewtemplate and </if> at the very bottom. 
  
Look at the other templates and if you need help on where to put the if. 
  
You can also add an else at the bottom to give them a message  like this adding any styles you would like.. 
<else /> <h4> you need 50 reputation points to view this forum..</h4></if>
The above if is for all forums for certain forums you can use this in the topicviewtemplate 
  
Below is for your selected forums by forum id number. 
<if test="enoughPoints :|:!in_array($forum['id'],array(47,54,75)) || $this->memberData['pp_reputation_points'] > 49"> 
The numbers represent the forum id numbers. 
<else />  <h1> 50 Reputation points required to view forum {$forum['id']} </h1> </if>