While any website has a right to earn revenue by serving ads, I feel that the companies who are ad-servers shouldn't be allowed to take over your computer. For example, when I'm reading an article on the Internet, then the next thing I know is I'm looking at another website (or the alert from my web app Firefox) because of a redirect from an advert. How is that a good thing and what can you do to prevent it?
The only good thing about it perhaps is we can all learn something about how slick someone else can be. But to prevent it, I look towards OpenDNS and Firefox to help me. You see, if you can find out where the ads are being served from (perhaps adserver.annoying-ads.com), using OpenDNS you can block access to your computer from anything coming from annoying-ads.com.
A good place to start is under the Tools menu option in Firefox. Select Page Info, and then choose the Media button. Look for the website addresses that look unfamiliar and with files that might end with ".swf". Practice with those first since these usually take over control of your webpage and do things like redirect your browser.
Each time you visit frequently a website that serves ads like this, you should notice a decrease in it happening to you. And if you go to other sites using the same ad companies, you're still covered because you've blocked that ad company's ads. Well, good luck with this method!