Hi,
this is a very good question and I would like to "expand" the question to "...not only web based infopath forms..." but forms for the IP client as well ... and in both versions of IP (2003 & 2007)
Actually I "listen" ;-)) to this in the "Signatures"-changed event --- but I would prefer to "...disable or remove the 'remove digital signature' link..."
Regards, Karsten