Unfortunately, a lot of copyrighting is going on; especially online. It's a difficult thing to get around. And, there are those who are talented enough to do copycat patterns just by looking at a pattern.
One way to protect yourself in the future is to write out your copyright on each and every pattern you put up online, in writing, or anywhere it may be in print.
Each designer has his/her own set of copyright rules. Some allow the pattern to be used for making and selling of items from that pattern. Some will allow making items from that pattern only for personal use or gift giving. Some allow making items from that pattern only as charity gifts; etc. But, ALWAYS state that nowhere should anybody duplicate your pattern. To change the wording, change the color of yarn, the size of the hook, etc does not warrant that they've taken your pattern and changed it into something new. It's still copyright invasion and punishable by law. (I state that almost word for word in nearly all of my patterns). Some designers even ask that if they do make and sell items made from their pattern to please be kind enough to include a tag that has the URL of their blog on it so as to help them generate more business. I don't think that's too much to ask for.
But, it's NEVER okay to copy a person's pattern to your own web site; especially if trying to pass it off as being your own pattern. Way not cool!
So, in the future, keep in mind that it's better to start off by protecting yourself in the beginning. There's also been some very good advice noted here as to contact the person first, then if there's no response, you can contact the person's ISP. One thing that I've always heard that is ALMOST as good as having a pattern individually copyrighted "legally" is to post in the mail a copy of that pattern and then do not open the seal unless there's a discrepancy such as this. Some courts will recognize this as a manner in which to show legal ownership of something by a certain date. Once you've had to go to court over an issue, it's already been decided (in a court of law) that a specific pattern belongs to you).