When using egg shells to deter slugs, you MUST crush them to nearly powder, if you leave them as just somewhat squashed (such as trod on or mashed with a fork), there's something in the liquidy-resin-inside-stuff (I didn't do chemistry, can you tell?
) that attracts them, rather than deter them.
I'd suggest, if you have a serious slug problem, eat sweetcorn (Corn on the cob). When you're finished, don't throw out the cobs, but leave them in the middle of your lawn or garden, amongst your veg, or wherever they are spotted most. During the night, go out with a pair of scissors (a head torch also helps), and you'll find that the slugs have all moved to the corn to feast, and all you have to do is snip them (clean in half) dead. Don't forget to check the area immediately surrounding the cobs, as there may be more slugs about.
Whilst this may seem extreme and a lot of hard work, it is very satisfying, as you know that you're catching slugs, and each one that you kill will not be eating your plants any more! If you do this every time you have sweetcorn, or possibly every night for about a week, then most of your slug problems should be gone.
Worms are good for the soil and plants etc, as they help to produce compost. I don't see why anyone would want to deter these from their gardens..? :S