Ioana, it is a tough one to combat. I have tried many methods. As the consequence, I probably made more damage than leaving them alone. There are several chemicals, which can reduce the population. But the complete elimination is tough. At the end, I had to do complete repotting with careful washing of roots, and spraying high concentration Physan-20 to the grow area.
Among the chemicals, Sevin, as Leafmite suggested, seems to be most effective (among easily available). I mix 1.5Oz/gallon of Sevin with Physan-20 (1 tsp/gallon). These 2 chemicals attack different aspects of snails. Then it would be also good to add Neem oil to the tank mix. An experiment by Dr. Hollingsworth's group showed that neem oil make the snails hungry, so they walk around more, and get exposed to the chemicals. You can spray and drench this all over. You'll need to repeat the treatment every 3 weeks. I think 2 weeks are the time for the eggs to hatch.
I do not see any phytotoxicity from Sevin. Some people are scared of Physan-20, and it is true that some weak plants could be influenced by it. And I can see that there is a possibility of mild retardation of growth. I'm not too concerned for my orchids at this interval (every 3 weeks) and this concentration. But it is probably better not to use it too frequently.
The other way is to put the mounted orchids under the water. It is better if you can let the leaves stick out. The snails will start to crawl out after a couple hours. But you have to be careful. I forgot about it and I let them soak for longer than 24 hours. Most plants get suffocated, and were dead. I think 6 hours or so is the appropriate time.
There is a paper (by Dr. Hollingsworth's group) suggesting 1-2% caffeine is an effective control. Never try this. I tried it since the paper said that they didn't see phytotoxicity. In 1-2 days, the roots stopped growing, and many of the orchids died soon. Caffeine is known to suppress root growth, and it is highly toxic to orchids at this concentration. Phalaenopsis were severely affected, and I believe many Cattleya could survive (retarded the growth for a long time).
Here is a paper related to this topic.
http://www2.ctahr.hawaii.edu/oc/freepubs/pdf/mp-1.pdf
Estacion, actually, their experiments (which is in a different paper) showed that current snail baits don't work so well for bush snails because they don't get attracted to it. If the snail touch it by accident, then it could kill it, but it isn't effective without a proper attractant.
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