Coordinated fake reviews detected

A seller's listing rockets up your marketplace ranking after a wave of 5-star reviews. Each review is individually plausible (different accounts, varied text, real-looking purchase history) and none trips your spam-text filter. But trust & safety suspects a paid review ring. The accounts reviewing this seller also co-review a small cluster of other sellers, were created in batches, and share subtle linkage (similar email patterns, overlapping IPs at signup, reviews posted in tight time windows). Volume is low, only ~300 reviews, so no rate alarm fired. How do you triage, confirm manipulation, contain it fairly, and harden ranking against it?