Use User Restrictions tables instead, line shortcodes are a general rule which applies to all users on site A.
Force all users not allowed to dial any extension on the remote (B) site with the user restriction table and add one or more shortcodes barring the remote numbers.
Create another user restriction table for all users allowed to dial one or more extensions and put the appropriate shortcode in that table.
And in the syste m shortcodes delete all shortcodes pointing to the Qsig outgoing line ID.
If all users numbers on site B are in the range 2xx theb create a shortcode in the first user restriction table : SC=2XX, TN= none,Action=Barred
and in the econd User Restriction table add a shortcode SC=2XX, TN=. , Action=Dial, LineID=the outgoing line ID of the Qsig trunk.
If you don't know how to do it then call someone who does if it is of that importance to you.