Competing Website Owner Libeled me. What can I do.

[tickerpop]

What can I legally do about someone libeling me, and potentially costing me valuable customers, and possibly losing the business altogether because of these false remarks.
I run a successful website dedicated to helping University of Phoenix Students with their homework. A couple of weeks ago from the info that I gathered a new site emerged based on my site that does basically the same thing. I am not sure why he or she felt compelled to play dirty so to speak, but they did. Left messages on my blog saying this:
PURCHASED A COM 140 TUTORIAL AND ENDED UP GETTING MY CARD CHARGED 100.00 I MESSAGED THE ADMIN AND THEY NEVER REPLIED. I LOST 100 BUCKS HERE.
And then immediately after:
ATTENTION ALL STUDENTS DO NOT I REPEAT DO NOT PURCHASE TUTORIALS FROM AXIATUTORIALS.COM THEY ARE ALL PLAGIARIZED AND ARE JUNK. DO NOT BE FOOLED THIS SITE IS BOGUS!!
I know that it was the owner of the site that is in competition with me because of the IP address that I was able to look up, and compare with the actual owner of the site based on his contact me page and the location with which they reside.
I emailed the site owner and asked him to log onto my site and retract the false comments that he posted, and that I knew it was he or she that was doing it because of the location etc, but I haven’t heard anything back from them.
Basically I would like to know what I can do about this. I have worked very hard over the last year to build this site up and get it where it is now, and for someone to come along and basically try to take it all away from me because they have no respect for others just doesn’t seem right, and if there is something legally I can do to stop this I would greatly appreciate how I would go about doing this.
Thank you in advance for any help anyone of you might be able to give me.