Read your contract. Perfectly fine for them to do so.
Quite simply, most every consumer services contract written is null and void ab initio due to them containing adhesive clauses. Adhesive clauses are clauses that state something to the effect of, this contract/agreement was created by both parties, meaning that when you sign it you are agreeing that you helped draft the damn thing. Of course this is almost always false.
You won't get anywhere with a customer service person citing this (or a vice president for that matter), but if you were to take a dispute over one of these to the mat, you will probably prevail in court.
A contract of adhesion is a, take it or leave it contract, i.e. "These are our terms, if you don't like it, hit the bricks!"
In court these do not hold up because there is no meeting of the minds, which is the primary quality of a valid contract.
So let's say you bring suit to Verizon and win, do they unblock 4chan? No. you get a refund. So they can block it; you just don't have to put up with it, but to say it's "perfectly fine" for them to do so is false.