Only Accept A Vote Once

How do I make it so that you can only vote once, and if you try to vote again you get an "already voted"
I've tried to do this, but I keep being able to vote over and over again.

thanks.
Derek Brown
http://www.emergeweightloss.com

I would also like to know how to do this. I have searched the forum, but I didn't find any solutions.

What I did was went into the admin panel and into the general settings. There I went near bottom of the page under Miscellaneous to Check
and selected IP Table from the drop down instead of no checking. I also left the locking timeout at 2 hours since I didn't think anyone would stay more than two hours on the page.