So this is the code to remove Posts of a specific Category from the post nav:
add_filter( 'get_next_post_excluded_terms', 'tu_exclude_terms' );
add_filter( 'get_previous_post_excluded_terms', 'tu_exclude_terms' );
function tu_exclude_terms() {
return array( 10, 15 );
}
the 10
and 15
are the Category Term IDs. you need to swap them to the category(ies) you want to exclude.
If thats not working, please share a post to where i can see the problem.