causality0 was making statements about his credentials and expertise, not his personal characteristics. I think ad hominem definitionally has to be irrelevant. “x knows nothing about topic y” is not ad hominem even if it is a false statement.
But like contravariant said, that doesn't mean the argument is wrong—just that it's a weak argument.
Really a fallacy represents a particular weak argument. The way to defeat them is not to name the correct fallacy but to point out the weakness.
If that doesn't work then you likely have the wrong fallacy, or the argument isn't fallacious to begin with.