what ARE you trying to remove exactly? title, meta, content, sidebar, etc? if you can tell us the OUTPUT that you want to get rid of we can help you backtrack to where it is coming from.
why would you need to remove 'thematic_abovecontent()' ?? it is just an empty hook. unless you have added something to it in your own functions.php it does nothing. think of hooks as a hat rack... they are only there for you to hang your hat on (add functions to).
if you HAVE put a function on thematic_abovecontent, but DON'T want it to display in this particular category then you need conditional logic on your initial function and NOT to remove the hook in places.
adding filters is different in that the function is already in place and you are passing it a different variable. i dont have a good analogy for filters and honestly, it took me a while to really understand them myself.
here is a basic example (bacon-free even)
function my_excerpt_length($length) {
return 20;
}
add_filter('excerpt_length', 'my_excerpt_length');
now in the wp_trim_excerpt function in the wp-includes/formatting.php file there is a bit that says
$excerpt_length = apply_filters('excerpt_length', 55);
basically this is going to take the value of that you returned above (with the my_excerpt_length function) and that will become the new value of the variable $excerpt_length. if you don't add a filter there it defaults to 55. maybe this one is like WP was playing with a soccer ball, but you passed it a basketball and so now it will play with that.
