That is the one drawback with the open structure area load... it applies that pressure to the exposed area of ALL members, not just the members in that plane.
But, here is a quick work around:
1) Apply the open structure area load and solve for that load case. This will create a 'transient' basic load cases containing all of the distributed loads you need.
2) Go to the Basic Load Cases spreadsheet and change the category of the transient load case. This "captures" the transient loads and you can now change them just as you would any other loads.
3) Display these transient loads. Then use the graphical selection tool to un-select all the members on the perimeter.
4) Now you can use the Delete to graphically delete the "all displayed loads".
Really, we just need to give users an option for that "open structure" area load to apply to only members in the plane of the area load. After all, the members in non-perimeter frames may be allowed to use a reduced pressure based on shielding or such.