Handpick hornworms from infested plants (this is the most effective means of managing them). They are fairly easy to find because of their large size. Just throw them into soapy water to kill them.
Their natural enemies are parasitic wasps which lay their eggs on the hornworms back if found, such worms should be left in the garden so the emerging wasps can parasitize other hornworms.
Insecticides are not usually necessary, however if the above options are not effective you may consider applying a product. We recommend Garden Safe Insecticidal Soap Insect Killer or Garden Safe Bt Worm & Caterpillar Killer.
At the end of the gardening season till the soil to destroy over wintering larvae. Repeating again in spring.