One good way of killing tomato hornworms is by handpicking them and drowning them in a bucket of soapy water. Another way to get rid of hornworms is by spraying tomato plants with Bacillus thuringiensis, which is bacteria that kills hornworms, but does not harm beneficial insects.
Alternatively, use botanical pesticides, as they are just as effective.
Attract braconid wasps and insectivore birds to the problem area to help get rid of hornworms. Use chemical insecticides when dealing with a large infestation of tomato worms. Another effective method of killing hornworms is by rototilling the soil to kill the worm's pupae buried underneath.