Yes, that would have been a good time.
How does this undo the months of work that you put in? Unless that work was specifically set up with agenda of "How do we get motownhorn banned" (and I suspect it wasn't), then the work is about how do you respond to him. That work would have still been relevant if you simply agreed with the owner of the site to put him on ignore.
In all that time working with bob did you ever ask if bob had talked to immamac and got his buy-in? If not, that is the risk you took. I agree it sucks that it happened but to use your sales example, if my boss never actually cleared the promotion / metrics for success with his/her boss then it's possible to get screwed.
No matter how aggrieved you feel, at the end of this two-month process, it played out that all you had to do was agree to ignore motownhorn and nothing further would have happened. So yes, agreeing to ignore him when immamac told you do would have been an appropriate time.