Here's one really dumb way to do it.

Have the exec cgi output the start of an HTML comment tag, and have the end of an HTML comment tag output at the bottom of the page.

HTML still arrives, but they don't see it.

Seriously though, why can't you have the whole second half of the page output by an exec cgi? That would solve your problem.
--

               ($_='
jjjjjjuuuuuuuuusssssssssttttttttt
annnnnnoootttttthhhhheeeeeerrrrrr
pppeeeerrrrrrrrrrrrllllllllllllll
haaaaccccccckkkkkkkeeeeeeeerrrrrr
                ')
               =~y
            /[a-z]//s
             ;print;
Comment on Re: Stop loading of HTML