You can definitely unpin it. Just click the green and white "pin" icon next to the title to unpin a pinned topic/thread. Click it again to pin the topic/thread.
Pinned topics will have the pin pointing down. Pinned topics will display at the top of a category (e.g. Beginner Questions, Trading Discussion, Community).
Unpinned topics will have the pin pointing up. Unpinned topics will display in regular order a move up and down the topic/thread listing as activity within changes.
Also, there's an option in your user preferences to automatically unpin a pinned topic/thread once you've read to the bottom of the thread.
To change that option, head to your User Preferences (gear icon) and go all the way down to the bottom of the page.
To automatically unpin topics, place a checkmark next to this setting.