If you open the Universal Email folder, you will see a Templates folder.. in there you will see where the body of the email is built.
It has a line that looks like:
if ($pval != "" && strpos($pkey,"Security_") !== 0 && $pkey != "x" && $pkey != "y") {
This should already remove any fields that begin with "Security_"... if you want to restrict more fields you can add them to this list, so it becomes something like:
if ($pval != "" && strpos($pkey,"Security_") !== 0 && $pkey != "x" && $pkey != "y" && strpos($pkey,"SimpleContact_submit") !== 0 && strpos($pkey,"Contact_Security") !== 0 && strpos($pkey,"Contact_nbsp") !== 0 ) {