As I have said many times in many threads, the USDA's best weapon is mis-information. Some are well meaning do-gooders, others are jack-boted thugs. They did not publicly declare war on Canas until mid 2006. Make them show you the authority that they are operating under in writing.
http://a257.g.akamaitech.net/7/257/2422/01jan20061800/edocket.access.gpo.gov/2006/06-3297.htm
To me this says if you got them before this date you are in the clear and they have no right to take your snails. Ownership of these snails in most states is legal, just shipping accross state lines is not allowed.
If you need to give them your snails, you can drop them off. You don't need to invite them into your home.