Poisoning the well (or attempting to poison the well) is a type of informal fallacy where adverse information about a target is preemptively presented to an audience, with the intention of discrediting or ridiculing something that the target person is about to say. Poisoning the well can be a special case of argumentum ad hominem

https://en.wikipedia.org/wiki/Poisoning_the_well

If you can’t attack the message, attack the messenger.