Add the following to the top of the Posts page:

<script type="text/javascript">
/*<![CDATA[*/
function aas(value) {
[if:show_aas]
return "";
[endif:show_aas]
[elseif:show_aas]
return value;
[endelse:show_aas]
}
/*]]>*/
</script>

Replace your current avatar code with this (if you use a default avatar):

<script type="text/javascript">/*<![CDATA[*/ document.write(aas('[if:av]<img src="[av]" style="width: [av_width]px; height: [av_height]px; border: 0;" alt="[username]\'s Avatar" />[endif:av][elseif:av]<img src="http://example.com/default.png" style="width: 80px; height: 80px; border: 0;" alt="Default Avatar" />[endelse:av]<br />')); /*]]>*/</script>

Else use this code:

<script type="text/javascript">/*<![CDATA[*/ document.write(aas('[if:av]<img src="[av]" style="width: [av_width]px; height: [av_height]px; border: 0;" alt="[username]\'s Avatar" /><br />[endif:av]')); /*]]>*/</script>

Replace the current signature code with the following:

<script type="text/javascript">/*<![CDATA[*/ document.write(aas('[if:sig]<hr />[sig]<br />[endif:sig]')); /*]]>*/</script>

Finally add the following to the account page:

<tr>
<td class="tableHeader"> Disable Avatars & Signatures:</td>
<td>
<input type="checkbox" name="show_aas" [if:show_aas]checked="checked"[endif:show_aas] /></td>
</tr>