Use to be the Thread Functions or whatever the button was called in the top right on the original post that provided a drop down and the close thread option.
EDIT: Looks like this is how you do it from the Sub-Forum page. Click the check box to the right of your thread listing, click the Edit icon (paper with a pencil picture) and then click close topics. I didn't actually close my thread so I am some what assuming this is it. Hope this helps
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