You cannot control who he calls through the facility's general phone system, but you can control what you pay for. If you fund an InmateAid discount phone number, that number works for calls to you only. He cannot use it to reach anyone else. Whatever he does on the facility's regular system with money from other sources is outside your control, but at least you know your money is not financing it.
Beyond that, the only real leverage you have is deciding what you are willing to keep paying for. Commissary, email, and phone accounts are all voluntary on your end. If the trust is gone, it is worth asking yourself honestly what you are supporting and why.