Most browsers will show you the URL of where you are going before you visit it by mousing-over the link. What you will need to do is mouse-over the "Reply" button on the post to which you wish to link to. If, for instance, you are using Firefox, you'll see the URL displayed at the bottom-left side of the screen (shown below).
(Here's the bottom left side blown up a bit in case the image above is hard to read:)
The number at the end of that URL is a post number, and you can append it to the end of the thread URL to link to the post. The trick is that you have to put #M in front of it.
So, for example, in the thread I'm using in the screenshot, the link to that particular post would look like this: http://community.webroot.com/t5/Security-Industry-News/Flash-Player-11-5-502-146/m-p/20788#M758
Best answer by JimM
For that, click Topic Options on the topic you're being notified about, and click Unsubscribe. Topic Options is at the top left side of the first post in a thread.