Importance of “nofollow” Attributes:
At their most basic, nofollow attributes control the flow of page rank between sites. When you allow your link juice to flow through to another site, a search engine takes that as kind of a personal endorsement from your site. So is it necessary add nofollow attribute to stop flow of page rank from one site to another. There are two methods.

Manual Method of Adding nofollow Attribute:

The nofollow attribute means you are simply inserting rel=”nofollow” within the HTML when hyperlinking to another site:

<a rel=”nofollow” href=”http://www.yourlinkhere.com” target=”_blank”>Link Text Here</a>

Add nofollow Attribute Using Plugin:

Install the “NoFollow” plugin. The plugin will add a check box to the “link box” that pops up when you wish to add a link to the highlighted text.  When you check the box, the rel=”nofollow” code is automatically added to the HTML with the link.

You can get NoFollow Plugin from here