Repelling Tomato Hornworm

What herb do you plant around tomato plants to keep worms away?

We presume you want to repel the tomato hornworm, the tomato eating larva of a sphynx moth that visits night flowering plants for nectar much like a hummingbird does during the day.
Basil is supposed to repel or confuse insects trying to locate tomato plants.
Dill can be interplanted as a trap crop. A 'trap crop' is one which attracts pests, luring them away from the crop that you want to protect. Because tomato hornworms are so large and easy to spot, it is easy to pick them off of dill plants. If picking giant worms off your plants is not your idea of a fun gardening experience, you can spray the dill plants with a Bacillus thuringiensis (Bt) spray.