This post explains how you would create a segment in Google Analytics that excludes traffic from a specific site. 'First, click Advanced Segments drop down and click 'Create a new advanced segment'. From left menu, drag Source into working area. Select condition 'Does not contain'. Enter name of site from which you want to exclude traffic. Then name and save segment. To apply segment, click Advanced Segments drop down again and select your newly created segment. (You'll see it under Custom Segments.) In this case, I named this segment "exclude promo site".
Take a look at the graph below and you'll see why this was a smart idea. The blue line is all traffic. The orange line is all traffic except traffic from promotional site. The orange line also shows a spike, even though it doesn't include any referrals from the promo site. This's the power of exclusion: If you want to find out how effective something is -- whether it's a traffic source, a promo, or a campaign -- try excluding its influence from your data. You might be surprised at what you find.