best approach to delete google reviews?

If your report gets turned down, don't stop there. Google has a review management tool where you can see the status of reviews you flagged and appeal a decision. The clearly fake ones sometimes come down on the second go.

You can't delete them as the business, only the person who wrote it can. What you can do is report it from your Business Profile if it actually breaks Google's rules - fake, off-topic, abusive, a competitor, someone who was never a customer. Bad-but-honest reviews won't come down however you word the report. For those, reply calmly and publicly, because people reading it judge the reply more than the one-star.