Closing/locking a topic is only by a Admin or moderator on the board in question.
If you are talking about here you can't.
If you are talking about your board.
Then it would be when viewing a topic select the tab at the top of the topic called "manage topic".
Select lock topic.
This will not allow anymore replies.
Or if the online moderation shows at the bottom of the page then select the topic and choose "close topic" in the drop down box.
Not sure why one method calls it lock and the other calls it close. But they do the same thing.
