Handpick hornworms from infested plants (this is the most effective means of managing them). They are relatively 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 sufficient, you may consider applying a product. We recommend Bonide Insecticidal Soap or Bonide BT Thuricide Spray RTU.
At the end of the gardening season, till the soil to destroy overwintering larvae. Repeat in spring.