I once bought an '89 Supra Turbo that had been sitting next to a field for a few months due to a blown head gasket.
Towed it home and started cleaning it up. As I hosed off the engine bay a single large black widow slowly crawled out on top of the turbo. I killed it and was glad to be done with that, or so I thought. I absolutely hate spiders.
With the car in my garage up on 4 jack stands I noticed each morning, strong, abstract webs reaching from the car to the ground. Crap, more black widows. Every day I would find more webs and kill a few more. Not a fan, especially while working under the car.
I finally posted on the Supra Forum where someone suggested to "bug bomb the S*** out of it!" So that's exactly what I did. I sealed up the garage and placed two bug bombs under the car and let 'em go. Never saw a live one after that.
All told, I killed 26, yes, 26 black widows on that car. They had just made it their home, lucky me.
Anyway, bug bomb was the answer in my case, and what I should have done that first morning in the garage.
Best of luck,
Justin