For example, if someone says "your favorite president is an idiot", it's an attempt to discredit and or disqualify anything and everything they say (and strongly implies that you're also an idiot for choosing such a buffoon as your favorite president, the guilt-by-association fallacy).

It isn't specifically aimed at undermining a particular "argument" or "position" they might espouse, it is a general dismissal of EVERYTHING they've done and or might do in the future.

Saying, "your favorite president is an idiot" is an ad hominem attack (both a direct and indirect attack).

It's also a broad-brush fallacy.

It's also a bald-assertion.

It's also an appeal-to-ignorance.