Poisoning the well
Poisoning the well is when before someone even speaks, you say they're not to be trusted or that they always say X, so people don't listen properly. You're trying to turn the audience against the person so that whatever they say is discounted in advance. It's a pre-emptive ad hominem. The fallacy is to dismiss someone's future claims because of who they are or what you assume they'll say, rather than judging their actual arguments.
Examples
Before you listen to the school council rep, remember they just want more break time and will say anything.
Of course the union will say that—they always want more pay.
She's from marketing, so she'll just spin it.
He's a lawyer—he'll find some loophole.