Poisoning the well
Discrediting a source in advance, so that nothing it says will be believed.
How to spot it
Is someone being warned off before they've said anything, rather than answered after?
Example
“Before you read his article, remember who pays him.”
How to answer it
Ask what is wrong with the claims themselves, once you've read them.
Related
- Ad hominem: Attacking the person making the argument instead of the argument.
- Genetic fallacy: Judging an idea by where it came from, not by whether it is true.