Baking the substrate will kill any mites in it, but it will not harm the mites that are INSIDE the snails. Although single snailmites can be found in the substrate, the vast majority of the population lives inside the lungs of the snails.
Baking will not harm them.
Even predatory mites can hardly get to those snailmites living behind the breathing-hole.
A successful treatment may take weeks, and sometimes the snails do not have that much time.
I once had three "Cornu aspersum" sent to me, they were covered with snailmites "Riccardoella limacum".
The snails were alredy severely weakened, i needed something quick!
I've found out that the fumes of an antspray killed the mites almost instantly.
The snails had nothing to lose so i took a glass-bowl, sprayed the antspray inside and put the bowl over the snails.
They showed no aversive raction, no retracting of tentacles.
I let them inhale the fumes for a minute or so, and when i inspected their breathing-holes, all mites seemed to be dead.
In the following days, the snails became active again and the mites never returned.
Later, I had mites on "Helix pomatia", same treatment: mites were instantly gone.
Some people reported that not all of the mites on their snails were killed. They had to repeat the procedure.
The reason for this failure is unclear (diffent mites?).
Most importantly: no-one ever reported any negative effect on the snail's health caused by the spray!
Even if this treatment may not always be effective, there is a good chance that it will help your snail.
I do not know how to insert images here; i have posted the "abouts" in a german-speaking snail forum.
The image there might help to get the right spray:
www.schnecken-forum.de/phpBB3/viewtopic.php?f=4&t=27545&p=313895#p313895
