@julia
One extreme alternative would be to reduce your garden to just bare soil (or plain lawn) for a year, no pots or planters, or whatever is attracting them and hope the slugs either die off or migrate away.
I heard of one tale, where a gardener with a snail problem used to pick them up and deposit them in woodland, a mile or so away from home but, he thought, fresh ones were taking their place. As an experiment, he painted numbers on one night's batch and, to his surprise, his labelled snails reappeared within days of release, having trekked all that distance to do so. I struggle to accept that the mollusc brain can support a homing instinct but today's news about monarch butterflies navigating across a continent with only a simple neural 'circuit' then maybe this snail tale isn't so far-fetched, after all.