You need to have some actual willpower.

The purpose of the nail polish isn’t to make it impossible for you to bite your nails - it will remind you that you should not be doing it, then it’s up to you to actually stop. Once you have some success you can trim them neatly and then it becomes much easier because you won’t want to ruin them.

I speak from experience as someone who managed to break a 30-year habit in 2020 - frequent hand sanitisation certainly helped!