They very well may be ignorant, but calling them that will just cause them to dig in their heels and double down on their position. It's not an effective tactic for convincing someone that they are wrong.
I can't say that I've ever done that-"called" someone "ignorant."
I'm sure I've said that their post seemed ignorant to me, and that's a difference that seems to have escaped people most of the time.
In the end, what frustrates me most is trying to make a point with facts and evidence, and having them dismissed.
That, and being called names...... (see that? I miss the "flaming mad" smiley, too-this fella looks like he's just constipated....)