[kh99]

Edit the FORUMHOME template. Near the bottom, find this block:

		<a id="sidebar_button_link" href="#">
			<vb:if condition="$show['sidebarposition'] == 'left'">
			<img id="sidebar_button" src="{vb:stylevar imgdir_misc}/<vb:if condition="$close_sidebar">tab-expanded-left.png<vb:else />tab-collapsed-left.png</vb:if>" alt="" />
			<vb:else />
			<img id="sidebar_button" src="{vb:stylevar imgdir_misc}/<vb:if condition="$close_sidebar">tab-expanded.png<vb:else />tab-collapsed.png</vb:if>" alt="" />
			</vb:if>
		</a>

and either delete it or surround it with <vb:comment> ... </vb:comment>. This takes away the control, but I think if someone already has the sidebar collapsed it will still be hidden. I'm not sure if that can be fixed by editing a template - you may need a plugin for that.