Negative Reviews Due to Indian Seller Hijacking

My listing in the UK had a rating of 4.6 with over 500 reviews. However, it has been hijacked by an Indian seller for over a year. The quality of their product is poor, and they even include colors I don't offer. Negative reviews about their product's quality are being left on my listing, causing my rating to drop from 4.6 to 4.0.

Since I don't have a registered trademark in India, I can't report the infringement. What should I do?