Trying to exclude posts from a category on the home page

Technically, pre_get_posts is an action, not a filter, and you don’t need to return anything and $query is passed in by reference, but that code should nonetheless work. Cleaned up it would be:

function excludeCat($query) {
    if ( $query->is_home ) {
        $catid = get_cat_ID('watch-isatv');
        $query->set('cat', '-'.$catid);
    }
}
add_action('pre_get_posts', 'excludeCat');

There are several potential points of failure though.

1. You are adding the filter after the main query has run. You must add
   that filter before the main query. functions.php should work but
   adding that code in your other theme template files, like
   archive.php or home.php won’t. The main query has already executed.
2. $query->is_home isn’t what you expect or need it to be. Add
   var_dump($query->is_home); to your code and load your page(s).
   Make sure it is true where you need it to be and false
   otherwise.
3. The slug, not the human readable name, used with get_cat_ID() is
   incorrect.

I’d put my money on #1 being the problem.