How would you have handled this big of defiance on the part of an 18 month old?

My son was sick yesterday so I went over to help out with his two kids. His daughter, Adrionna, is 18 months old, and Jaden is 5. The kids and I were out front. I was sitting on the porch and Adrionna was running UP and DOWN this slight rise and dip in the yard. She was practicing her mountain goat skills.
The she wandered over toward the drive way. I gave her a verbal caution…“Adrionna! Don’t go in the drive way!”
She gave me a cheeky little grin and disappeared behind the car. I got up, intercepted her and chased her back to the yard, then got down and spoke to her sternly. She looked up at me with big, solomn blue eyes, then gave me that cheeky grin and and hugged me. The little flirt! Doesn’t work on Gramma, and fortunately it doesn’t work on Dad either! But she’s been a flirt since she was a baby!
A few minutes later she headed for the drive way again. Again paused when I said her name, and again disappeared behind the car. I intercepted her again, and this time reinforced my instruction with a light pop on her diapered butt.
THEN, a moment later, as I was watching, she walked up to the very edge of the driveway, pushed her little toes against the concrete edge, then slowly leaned in until she could put her hands on the car. BUT she wasn’t in the driveway! Another cheeky grin. “I gottcha on a technicality, Gramma!”
I was a bit at a loss. If it had been my own kids I would have addressed the sheer defiance of it! But I try to stay in line with how my kids address their kid’s moments of disobedience, so I did nothing because I didn’t know what my son would do. She walked up and down the length of the car, touching the car, but feet firmly inbound.
I told Chris about it later, cutting my eyes at him because he was the SAME WAY!
He said, “Hmm. Guess she’s too young for contracts.”
When my son was about 10 he actively started looking for loopholes in my instructions. The kids had household chores to do before I got home. I told Chris he had to do the dishes one day. I came home…the dishes were done, but the floor wasn’t swept and the counters weren’t cleaned!
He said, “You just said to do the dishes so I did ‘em!”
:)
After that it was a battle of wits. For the next 3 years, every time I wanted him to do something I’d write up a contract, which he had so sign and initial, explaining, in detail, what needed to be done. I had to work my brain to try and not miss anything, and he, in turn, worked his little brain trying to find a loophole! Which he did, more often than not.
It was like a game. It was fun and challenging, and I had to laugh every time he “got me,” .....but, then, him being him which is a very cool kid he go off with a grin to clean whatever he had missed. Of course then I added that to the contract. The “Cleaning the kitchen” contract was about 16 pages long!

Anyway, do you think her bit of defiance should have been addressed, in spite of the fact that she literally Stayed In The Yard, as she was told?