Just posted an upgrade so you can now show "disabled by the adminstrator" in their profile
to upgrade just
Open Member info and find
Code:
$userinfo[signature]
and replace with
Code:
<if condition="$userinfo[fieldXX]!= 'No'">$userinfo[signature]
<else />
<center>Signature Disabled By The Administrator</center></if>
and finally
Code:
replace XX with the field of the new profile field and your done