Last year, I had a nest underneath my back door and both myself and my puppy were constantly being stung. This year, I sprayed and patched the space under the door before they appeared. But, before I did this, I checked for a nest and could not find one. I sprayed anyway. Since they haven't appeared yet, will they because I sprayed? And if they do, how can I get rid of them before there is a repeat of last year?

Caulk, mortar or any other applicable patching is about the best you can achieve as far as preventing them goes.

Typically wasp sprays kill on contact and do little to prevent them in the future.

Look close for any other voids that you can fill appropriately (caulk, mortar, plastic wood, etc.) and you've done about all you can do... except for keeping your eyes peeled - especially now that they are starting to come out - so that you can nip it at the bud when they first begin building.

Hi,
I agree with the answer before this one. There isn't any Wasp Repellent that I know of.
Here in the Mountains of VA, we refer to them as "Waspers", which is just "mountain slang" for a Wasp.
About the only thing you can do, is to kill them with Wasp Spray (Raid, etc) while they are building a nest, so it won't continue to get any larger, and stop the building.
Best of luck.