The signature field by definition, should require you to enter a password when you sign the form with your digital certificate.
As well, Acrobat / Reader has built in logic which automatically validate mandatory fields by coloring them, before a submit event. On a signature field, you could cancel the sign operation using the following script on the preSign event of the signature field to prevent the form from being signed:
xfa.event.cancelAction = 1; // this will cancel the sign action. Requires reader 9.0 or later.
Example.
if (xfa.form.execValidate() == 0) {
xfa.event.cancelAction = 1
}
I would recommend upgrading though, since the 7.0 products have end-of-lifed with respect to support.