Try to sell them if they're getting big! Some people enjoy them quite a bit. If you don't want to sell them or give them away, take your inhabitants (minus the snails) out and put them in a temporary tank. Begin removing as many snails as possible. When you think most are gone, depending on your tank size you can either fill it with hot water and scrub it, or put it in the dishwasher! That should be good. For the decorations, it may be hard to remove snail eggs from them, so try and get new ones if possible. Hope this helps!