It could be removed from that tab too, would just be another template edit somewhere.
Blocking the homepage field though isn't going to prevent spammers. They will still register and they'll put a URL somewhere else, or in their signature, or start a new thread... All that blocking the homepage field will do is potentially frustrate some of your visitors who would use it legitimately.
I take quite a relaxed view of spam on my board. It happens, and it is quite regular, but that's why we have moderators. It's one of those battles you'll never win.
However, if you do want to effectively disable the input, or at least restrict it to more trusted usergroups.
account_personal_details template change:
Rich (BB code):
<dl class="ctrlUnit">
<dt><label for="ctrl_homepage">{xen:phrase home_page}:</label></dt>
<dd><input type="url" name="homepage" value="{$visitor.homepage}" id="ctrl_homepage" class="textCtrl" /></dd>
</dl>
to
Rich (BB code):
<xen:if is="{xen:helper ismemberof, $visitor, 3, 4, 5}">
<dl class="ctrlUnit">
<dt><label for="ctrl_homepage">{xen:phrase home_page}:</label></dt>
<dd><input type="url" name="homepage" value="{$visitor.homepage}" id="ctrl_homepage" class="textCtrl" /></dd>
</dl>
</xen:if>
Where 3, 4, 5 are the usergroups who are allowed to edit the homepage field.
By the way, I've not tested either of these, I'm pretty sure they should work, though.