Relaxasaurus

^ Unfortunately that won’t work.

query_posts('showposts=10&cat=67,3&p=-6,-12')

WordPress doesn’t understand p=-6,-12 (yet).

What you can do is…

query_posts('showposts=10&cat=67,3');

if(have_posts()) :
while(have_posts())  : the_post();

if (get_the_ID() != 6 && get_the_ID() != 12) {
    //display post goodies
}

endwhile; else:
endif;