YOu can't close it. You have to have a mod do it for you or if it is inappropriate they will lock it themselves. Try pming one of them and asking one of them to do it for you. They sohould be more than happy to help.
Our policy is that we don't close a thread unless it is getting or has gotten out of hand and cannot be brought back on track, and/or has lost the educational value due to the course of the discussion. If there are inappropriate replies, please use the Report Post (triangle with exclamation point button at the top right of every post).