Jim,
 
I saw the warning about leaving the shells in the sun but little help on the
smell and cleaning them.  There is no easy, clean, unmessy, etc. way once
the shells are in the condition you describe.  You have to cut off the operc
(bleach will eat it away), pull out as much of the animal remains as
possible and then immerse the shell in a solution of Clorox and water (1/2
and 1/2 is good).  Try to get the solution into the shell (i.e. get all of
the air out of the shell you can).  After a day or so, rinse under a strong
stream of water.  If lots of animal comes out you may be okay, but typically
you will have to repeat the bleach soaking to get further back into the
shell.  A Thais is small enough that some remains left inside will most
likely just dry up and present little problem.  You can also try the ant bit
but I favor the direct, messy way myself.
 
Good luck,
 
Tom Eichhorst in New Mexico, USA