There are few antidotes for the number of poisons available, so what is the treatment goal for a poisoning if there is no antidote?

If a particular poison does not have an antidote the next best treatment is to limit the amount of the poison to be absorbed.  Activated charcoal is often used to absorb ingested poisons. Brushing off powdered poisons or flushing with high volumes of water will block, slow or prevent absorption. Once the absorption has been controlled or contraindicated, you are left to treating symptoms.