You are doing something unethical. Your service should be stopped as soon as possible. Because you are trying to provide a service for which you can ban a legit account only for few bucks though the account might be too much important for the person who is operating the account and also whose followers might lose the service from that person.
I respect your opinion but i am far from being the first or only seller to provide this service. Besides, this is service like any other which can be used for good or for bad intents. I've been getting many requests to ban accounts that harassed other people either verbally or sexually. It is not the service that is unethical, it is what people do of it.