Then you have the opportunity to burn the Sybil attacker's funds in mining fees. Honest coordinators issue temporary bans to non-signer inputs as a DoS protection measure, so a convincing Sybil attacker would have to do the same. During this ban time, the attacker would have to continue to pay mining fees to remix some portion of the UTXOs that the target originally observed in the transaction as well as their ancestors and descendants. The more convincing the attacker tries to be, the more mining fees they have to pay. This costs the target nothing, and they can repeat it as many times as they want.