From my understand, you want to eliminate all signatures in new private messages, if I am mistaken, ignore my code and correct me.
To remove signatures in new PMs you must modify the correct file (pm_newpm). Search "pm_newpm" in your templates.
Then locate and remove the following code:
PHP Code:
<if condition="$bbuserinfo['signature']">
<div>
<label for="cb_signature">
<input type="checkbox" name="signature" value="1" id="cb_signature" tabindex="1" $checked[signature] />
$vbphrase[show_your_signature]
</label></div></if>
Note: Please backup all documents to ensure no information is lost.