yeah, the boiling works but it ends up needing to be done not quite all at once to keep your BB alive..which could end up just giving the snails a chance to infest whatever you've already cleaned. it would probably have to be like:
boil decorations, leave out of tank
boil substrate, leave out of tank
make sure inside tank is still free of snails, then take out the filter media, make sure filter is free of snails, and replace it the media.
replace media and decorations
though that would probably have to be done over a few days, and still leaves the chance for any hidden "snail egg sacs" to be still around [unless they're like a sort of livebearing snails? i don't know much about snails, just that i have a lot of them!!]. what i tend to do is if my tank is getting especially overwhelmed, i'll pick out whatever snails i can reach without sticking my arm in [i.e. the ones toward the surface, dur] and throw them in either to my puffer fish or my turtle. i can usually get a whole lot out this way, since at different times throughout the day a lot of them will gather toward the surface, and by the time i've gone around the tank once they've started piling up again. over a day or two this really cuts back my population, but in no way eliminates it!! mine all showed up the first time i added life plants from a tank into there [from a tank as in from a tank at the fish store, as opposed to the expensive guaranteed snail-free ones that come in a plastic container from petco]
shell-less snails? i've never heard of that. i'd ask about those in the invertibrates forum
Darn those snails!!! And yes, it is a HOB filter. I don't even know what kind of snails they are. . they look like they have no shells and are clear. I noticed them once I brought plants into my tank 
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