I see forums as debating clubs more than social gatherings.
If I exclude that person because of their political beliefs, then most others will perceive me negatively and exclude me from their social gatherings. I would become socially isolated. If I exclude that person because they're rude to everyone, then others won't care that I exclude that person from my social circle because they're excluding that person as well. This is basic human interaction.
What makes you so sure that will happen?