Argument by characterization
Argument by characterization is a rhetorical manipulation in which the attacker characterizes the defender's arguments as somehow deficient (e.g. partisan, ignorant, misguided, unoriginal, etc.) without actually showing how they are wrong, i.e. dismisses them.
It is an indirect form of ad hominem in that the defender is ostensibly not being attacked, but rather the defender's argument.This would be legitimate if it were the argument's substance that were being attacked, but in fact the argument is essentially being used as a proxy through which to attack the character of the defender.
It is arguably also a form of intimidation in that it implicitly threatens the defender with being similarly characterized ("your argument is foolish" carries an implied "and you are foolish if you continue to defend it") if they do not abandon their defense.