To keep the prefix, but remove the link that leads to the filtered results page, access the "thread_list_macros" template. At line 91, you'll see:
<a href="{{ link('forums', $forum, {'prefix_id': $thread.prefix_id}) }}" class="labelLink" rel="nofollow">{{ prefix('thread', $thread, 'html', '') }}</a>
Remove the link HTML from this line, so the result is:
{{ prefix('thread', $thread, 'html', '') }}
And that's all you need to do. You'll see that the once active link on the forums page is no longer active.
Hope this helps.