[Mr-Moo]

There is a built in feature in VB that allows you to tick a small checkbox and scroll to the bottom and select "close selected threads".

Other then that, you would need to edit the PHP file which designates the "move all threads". Once that file is found, you would need to copy that line and duplicate it. Once duplicated you will need to modify it to use the "close" function instead of the "move" function.

Let me know if you need further assistance!
Hope it helps,