All the precedents here are well established.
Explicit, credible calls for violence have never been protected speech. Parler’s own TOS forbid it. No moderation effort can be perfect, but we don’t need to argue in this case whether their moderation is sufficient. A violent insurrection was openly discussed and organized by thousands on Parler and then they went and did it. Lin wood called for the execution of the Vice President and many of his followers attempted to do just that.
In terms of the limits on a person or business to control with whom they will do business, utilities — ISPs in this case — should be regulated so they can only deny service for specific, limited reasons. They generally are, though I know this needs improvement.
Beyond that, businesses and individuals should have wide latitude to decide who to do business with.
What concerns me is the strange inversion freedom of speech has undergone in the Trump era. Instead of protecting the people from the government, it’s now invoked to protect the government — and supporters of the current administration — from the people. Now it means suppressing ordinary liberties like the right to not support people, content or speech you don’t want to.