Fake and paid for reviews

I recently received a 1 star review and looked into it, here is what I found.

In my niche there is a new competitor with five 5 Star reviews. All reviewers have names that come shall we say from a specific region. They all review both of my competitors two products and give both products 5 star reviews. And one of the reviewers gives my product a 1 star review.

All 5 reviewers have no history reviewing products. I think you get the idea.

So how to best deal with this ?