I think there's a usergroup option for this, but if that fails:
Styles and Templates, Style Manager, Edit Templates, Editor Templates, remove or comment out or block this in editor toolbar on and off:
<
Code:
if condition="$show['color_bbcode']">
<td><img src="$stylevar[imgdir_editor]/separator.gif" width="6" height="20" alt="" /></td>
<td><div class="imagebutton" id="{$editorid}_popup_forecolor" title="$vbphrase[colors]">
<table cellpadding="0" cellspacing="0" border="0">
<tr>
<td id="{$editorid}_color_out"><img src="$stylevar[imgdir_editor]/color.gif" width="21" height="16" alt="" /><br /><img src="$vboptions[cleargifurl]" id="{$editorid}_color_bar" alt="" style="background-color:black" width="21" height="4" /></td>
<td class="alt_pickbutton"><img src="$stylevar[imgdir_editor]/menupop.gif" width="11" height="16" alt="" /></td>
</tr>
</table>
</div></td>
</if>