You will need to find where they are coming from and block the holes.
Horrid things.
Get up in the middle of the night, whilst it is still dark and the house has been quiet for a few hours. You should be able to see where they are coming in from the slug trails inside and outside your house.
Use plenty of salt at this point, and cement up as soon as possible.
By airvent covers from your local diy shop - they let air through but stop insects and slugs.
They can get through amazingly thin spaces - if your door is not airtight, they may find a way to squeeze through, so check out for trails around the sides, top and bottom of any doors where they may get in.