I've found a few of the big slightly buried ones too. I really did go yuk when I pulled the first one up & saw all the white eggs below. Urgh.
I got rid of them all carefully. They were in pots that had copper tape on, but is a few years old. I did find it worked fairly well until this year though. Unless as has already been said they can get across from foliage in another pot or drooping leaves. It's expensive but so are most non pellet options.
We have a small pond & slug numbers have gone down a bit even though we don't see frogs, we get frogspawn each year.
Hedgehogs are meant to be great for eating them, but we're too well fenced to keep the dog in.
There are lots of plants I just won't grow because of them. I'm determined to get delphiniums established. I'm growing them on in pots in the table from seed until they're big enough to cope hopefully though.