How to restrict Next & Previous post links to category?

Hi,

In the posts at the bottom there are links to the next post and previous post. If the next post is in a different category from the current one, how do you get it to stop the next/previous link from showing?

I have tried putting this in the under content section in hook:

<?php previous_post_link('<h6 style="float:left;">%link</h6>', 'Previous', TRUE); ?>
<?php next_post_link( '<h6 style="float:right;">%link</h6>', 'Next', TRUE ); ?>

and also using css to make the default links invisible:

.post-navigation {
display: none !important;
}

.cat-links {
display: none !important;
}

The issue with this is that these links are visible from the page where you can see the exerts of all my posts. I don't want to display these links here because it looks cluttered and is not needed. Any ideas?

Much appreciated. Awesome software by the way!