Click on "Thread Tools" on the top of the page, then check the option "Close Thread" then click on "Perform Action" and it will be closed
Hello, it is also worth adding that you can close threads with posts by scrolling down to the 'Thread Management' section and ticking the box next to 'Close this thread after you submit your message'.
Yoda and eyes have basically covered it. You can click on "thread tools" at the top of your thread and close there, or close the thread by using Eye's method.
On the off chance the thread is in a board where you can't close your own threads, you can also report your first post on it requesting that it be closed by a moderator.