If someone is violating your copyright you have a couple of options. These apply when someone has copied the entire post.
1. Send them an email using information from their about page. Sometimes people find a post that they like ad don't understand that they can't copy/paste it. If it's a "real person" blog, this often does the trick, and you've kept the matter private and not embarrassed them.
2. Leave a polite comment indicating that you are the owner and originator of the post and ask that it be removed.
Allow a reasonable time for a response to the above options. If you don't hear anything further, you can go on to. . .
3. Send them a DMCA violation notice. You can find forms and instructions.
If the entire post hasn't been copied, only a few lines, it's not a DMCA violation. You may have had you RSS feed imported to a splog site. They are annoying and try to make money off the the feeds that they import, but since they are only excerpts and they provide links back, I'm not aware of any effective way to deal with them if option #1 and #2 above don't work.
Also, take it as a complement. Your articles may be so good that people are copying it because they cannot write anything better.