Are they a problem? An organic Molliscide is caffeine - keep your old coffee grounds or tea bags and make a up a 'brew'. Spray it over the surface.

While I don't know about these land snails I have had similar in an aquarium. I eventually broke the whole thing down and bleached the gravel, poured boiling water over it, and let it sit through a winter in the garage where the gravel became a frozen mass. Next summer I reset the aquarium and I still had snails. Absolutely incredible.
Since they were diminished though not beaten, I basically just continued as you are doing, plucking them out one at a time and keeping an eye out for eggs. Eventually they disappeared.

Sounds like bush snails. Kill them dead. Kill them quick.

Sam

Oh no! I really do feel for you! Snails are terrible! They will eat all of your plants if given half a chance, so you must declare war on them, which you have. I would get as many of those slug-killing pellets out of there as you can. You don't know what the build-up of toxins could do to your plants. Then bury a bowl full of beer into your substrate, so that the rim of the bowl is level with the substrate. At night, the snails will come out and go to drink the beer and drown. =) This beer trap is highly successful in gardens as well. Keep doing this until you have no snails left, and then keep doing it for a few weeks anyways, because there will still be eggs in the water/substrate, and they will hatch and also want the beer. Good luck! You will need it. . .

Or just replace everything in your terrarium!