You CAN block you competitors ads from viewing on your page (as many as 200, I think). Go to the Google "AdSense Help Center" and follow these links ... Google Help > AdSense Help > Working with My Ads > Filtering Ads
Here is the direct link:
www.google.com/adsense/support/bin/topic.py?topic=154
You actually need to block all the ads' URLs, so each competitor could have multiples. Be sure to check out the Google AdSense Preview Tool, it allows you to see what ads WILL be running on your site.
I hope it is worth it for you,
Duane Cosner