Is there a settings for this? I cannot find them. And if there isn't, how can I stop users collapsing my forum categories.
In that case it would best, in my opinion, to remove totally the collapse option then have it there and not allow the members to use it.
Anyway, find and remove the following code at the forumhome_forumbit_level1_nopost template:
<vb:if condition="!$show['collapsable_forums']">
<a class="collapse" id="collapse_c_cat{vb:raw forum.forumid}" href="{vb:raw relpath}#top"><img src="{vb:stylevar imgdir_button}/collapse_40b.png" alt="" title="{vb:rawphrase collapse_this_category}" /></a>
</vb:if>