Let alone risk involving your audience in the fight. Imagine every page all the comments are focused on this one user rather than what you spent hours labouring on. 100% best thing to do is block/ ignore this person.
As for motive, I was cyber-stalked on an off in a 4 yr period and this individual hyper focused on myself and a few other artists. Based on information that surfaced about them later, it seemed to be that they chose targets that their either perceived as currently doing what they wanted to do (comics) and/ or enjoying some sort of success in that field that they deemed unworthy of having any attention at all. So they spent much of their free time fruitlessly trying to convince our audiences that we had tarnished reputations, our subject matter and technical ability were actually terrible when viewed through the lens of a "True Intelligent Critic ™" and even trying to blame us for their false attempt at suicide. Eventually left me alone when I straight up left the community they were hate-scavenging on. (Albeit I left for different reasons.)
What makes me laugh in these situations is how much they shoot themselves in the foot: All that passion and time they spend on destroying could've been spent on actually getting somewhere with their work.