This would need some really simple plugin code that you may need to place in the showthread hooks. It is so simple code that I thought of sharing them here instead of making the plugin out of it.

Add the following code in the couple of hooks.
  1. showthread_start
  2. showpost_start
Code:
if(!$show['member']) 
{
print_no_permission();
}
And that should do the trick for you.
coderzone like this