It is a copyright violation. At the very least, you can send a "cease and desist" notice to the offending blogs. It's up to you whether you ask them to take the articles down, or tell them they can continue to use them as long as they attribute them to you (and include a link back to your website).
Some of them will comply right away. In many cases, the infringement is inadvertent. Many people just don't realize how copyright law works and think if they find an article they like online or in a newsletter it's OK to publish it on their own site.
Others will ignore you. At that point, you can file a copyright infringement notice with their ISP. With a little sleuthing you can usually discover who their web hosting company is. (If you're lucky they'll be somewhere like Blogspot, Blogger or Wordpress.com, which makes it a lot easier.) Tell them specifically which articles on the blog are infringing on your copyright, including the URLs of the infringing pages, when the article(s) were published by you and the efforts you've taken so far to resolve the problem. Some ISPs will have an online or downloadable form for filing these requests.
The ISP may give the blogger one more chance to take down the article(s) on their own before taking further action. If the blog owner doesn't take action (or if the ISP chooses to not give them one more chance), the ISP will then either take down the infringing articles or (more likely) take down the entire blog.
A few ISPs may ignore you. When/if that happens, you can choose to hire a lawyer to pursue them. However, most of these bloggers are stealing content from multiple other sources because they're not that good at blogging on their own, so they're probably not making that much money from their blogging efforts in the first place.
You can also at that point file a DMCA (Digital Millennium Copyright Act) request with Google and Bing. (Since they're both in the USA, this act applies to them.) While this won't get rid of the blog posts themselves, the search engines will remove the blog from their search results (and replace it with a notice that it's been removed for copyright violations). I have used this route with Google against a company that "lifted" one of my company's websites whole -- just duplicated the entire thing. Google was very responsive and had taken their site out of the search results within a few hours. (It may take a day or two for the update to propagate throughout all their data centers, but the original action took place very quickly.)
And, if you don't already, include a copyright notice at the bottom of every ezine article. It isn't necessary to have the notice there for the articles to covered by copyright -- that happens automatically when as soon as you publish them -- but it may help put others on notice that you are aware of and enforce your copyright.
Hope this helps!