I'm sorry they wasted your time and money. I think if someone disappeared for a month that would break the expectation of payment, but a small contract would settle that.
The irony is that the project had no value to me. I had already implemented that feature, and just wanted to understand if the candidate would be able to do the work I'd already done for myself. Indeed I picked the problem because I had solved it myself under timed conditions and knew how long it would take me. I wrote the code for the feature in about 20 mins and then tested and checked-in under an hour. I expected that a new person with less experience and skill than me would take about 3-5x the time, and that would still be okay. Even if they took a day that was not an issue for me. Which is why I paid them for one day of their time. They might have taken a whole day. But 30 days was somewhat too long. Thankfully I'd priced it at a fixed cost. So my downside was fixed.
Still the learning was not cheap. I was paying what would be considered consulting rates in India.