I'm a Research Fellow at Open Philanthropy, though all views I express here are my own.
Before OP I was an independent researcher in global health and biosecurity, and a Charity Entrepreneurship incubatee. I have an MD and undergrad degrees in philosophy, international relations, and neuroscience, all from the University of Sydney.
Thanks. Given Alice has committed no crime, and everything else about her is 'normal', I think organizers would need to point to her belief to justify uninviting or banning her. That would suggest that an individual's beliefs can (in at least one case) justify restricting their participation, on the basis of how that belief concerns other (prospective) attendees.
Yes I'm not saying anyone was - this is a thought experiment to see if exclusionary beliefs can be a coherent concept. We can stipulate that Alice has this sincere belief, but no history of such attacks (she's never met a Bob), and hasn't made any specific threats against Bob. It's just a belief - a subjective attitude about the world. If Bob does not attend due to knowing about Alice's belief, is that reasonable in your view?
As a light thought experiment, what if Alice's belief X was "People called Bob are secret evil aliens who I should always try to physically attack and maim if I get the opportunity" ?
Bob would understandably be put off by this belief, and have a pretty valid reason to not attend an event if he knew someone who believed it were present. Does it seem reasonable that Bob would ask that Alice (or people who hold the attack-secret-alien-Bobs belief) not be invited as speakers? Is that a heckler's veto, and contrary to free expression and intellectual enquiry? Is Bob's decision not to attend just a matter of his own feelings?
If answers to the above questions are 'no', it suggests it's possible for a belief to be an 'exclusionary belief', on your terms.
One might distinguish de jure openness ("We let everyone in!") with de facto ("We attract X subgroups, and repel Y!"). The homogeneity and narrowness of the recent conference might suggest the former approach has not been successful at intellectual openness.