The shill gambit is a fallacy where someone dismisses another party’s arguments by proclaiming them to be on the payroll of some conspiracy without having evidence other than the arguments themselves. The shill gambit is a type of ad hominem and poisoning-the-well fallacy. It shifts from addressing the substance of the argument to attacking the person and their credibility instead.
Flat-Earthers would usually commit the shill gambit when they feel cornered. They would accuse anyone outspoken against flat Earth of having been paid by the global conspiracy. We at FlatEarth.ws get these accusations a lot.