I have had shells cleaned many times by leaving them out in a place where
they were accessible to insects & other small creatures. As a matter of fact,
just yesterday I placed in my yard the shell of an Anguispira alternata that
had died in a terrarium to get it cleaned. Ants are not the only soil
creatures that would eat mollusc remains. Isopods (pill bugs), millipedes,
centipedes & hundreds of other arthropods & possibly even slugs & various
worms would also contribute to the cleaning as long as the shell is large
enough for them to get in it. There is no need to bury the shells. If the
remains are dry, wet them first, then place the shells -with apertures
preferably facing the ground- in a damp location away from direct sun light &
cover them loosely with leaves, etc., to retain moisture. Almost all soil
inhabitants (except, perhaps ants) require constant moisture to survive &
consequently, they are abundant in damp soil.
 
I would hesitate to do this with microshells, because ants may carry them
away to their nests unless some sort of net with holes large enough for ants
but too small for shells is used. Otherwise, even the largest shells can be
cleaned this way, although the smell of a large conch rotting in the backyard
with flies hovering over it could bring complaints from the neighbors. Also,
if raccoons or similar animals visit the place, it may be necessary to place
chicken wire or something similar around large shells to prevent them from
being taken away.
 
In a week or two there will be nothing left in the shells. You'd be amazed
how good a cleaning job those critters do.
 
A.