If you have phpMyAdmin open it. And then open the database, where the database tables of your Kunena Forum installation are.

Click on the left side on jos_fb_categories.

ON the right side above click on SQL and you will see a input textfield, where you can insert code. Delete the existing code in this Textfield

Update `jos_fb_categories`
SET `locked`= 1

and click on OK.

This will lock all forums and categories.

To unlock all again insert this:

Update `jos_fb_categories`
SET `locked`= 0