BT
Bacillus Thuringensis.

It's a stomach poison specific to larvae of Lepidoptera (which includes tomato hornworms). They eat a leaf with BT on it and it paralyzes their gut. They stop feeding and die. Since it's specific to Lepidoptera, it doesn't harm beneficial insects like bees and ladybugs. Widely used by organic farmers. Readily available in most hardware stores. Sometimes called Spinosad.

Tomato Hornworms are so big I can usually eliminate them by simply hand picking them off the plant. If you see a clump of little white eggs on the Hornworm's back, those are from a parasitic wasp whose larva will feed on, and kill, the Hornworm. So just remove and relocate those specimens. Otherwise, the BT is your most environmentally friendly choice. Oh yeah, when you pick a Hornworm off your tomato, it'll turn around and try to "bite" you. Intimidating but harmless.