I read once that if you covered grass at a time when the moon either was getting bigger or as it was getting smaller. I'm sorry I don't know the sientific words of the moon's looks changing. One of the times was a better time to cover grass to kill it. The other time the grass just grew under the plastic. I hope you know what I'm talking about. We want to plant pumpkins and watermelons soon and digging up the grass is to hard. Thank you.
Unfortunately grass will grow no matter what. Hot water and vinegar in equal parts will kill grass but the roots or stumps depending on what type of grass you have will remain. Once the grass is dead you have no options I'm afraid but to hoe the area and pull out the roots. Grass roots are deep, but pulling or hoeing them out should do the trick.