Baby powder - with some cayenne pepper mixed in just to give it some extra kick. You might want to consider investing in a lot of baby powder and spreading it all over the yard with the cayenne pepper, on a day when no rain is forecast for a spell.
I'd recommend diatomaceous earth, but I've heard (spurious) reports that it might be carcinogenic. My guess is that, because, as tiny fossilized animals, it acts like microscopic shards of glass, then if you inhale it, although it only feels like a sore throat, it creates more exposure to whatever carcinogens might be in your environment. If you breathe it in deeply enough, I guess it can get into your lungs and have the same effect - which, if this is the case, lasts only as long as the tiny cuts take to heal. I just use a damp handkerchief tied around my mouth (and I wear sunglasses to protect my eyes - must be quite a sight!) when applying it. Once the diatomaceous earth is applied and settled, it should not be a problem, unless wind or AC or whatever will kick it up into the air.
But I haven't taken the time yet to research whether or why diatomaceous earth is actually carcinogenic. Again, it's only fossils, so I don't have any idea why it would be.
But just in case, if you don't find out, yourself, stick with the baby powder!! You can always vacuum it up later.
Put some petroleum jelly under the crack of the door frame, and they shouldn't be able to get in after this :)
L.