To disable your chat box please go to your "Profile/Settings" area (which is the 3 line button next to the Community tab) and then under "My Info" scroll down the page till you see a box that is titled "Enable Chat". From there you want to make sure it is un-clicked or has no check-mark and this will make your chat box disappear.
If you would like to have your chat box reappear or back then just click on the box to check mark it and it will enable it.