Fungal spores are largely airborne. If an infected bed is nearby, they'll get there. Use a fungicide as a protectant. The fungicide won't kill the fungus once it is established on the plant, but can prevent it from getting established in the first place. I don't think there is any way for residential users to disinfect their soil. You're stuck with it.

As discussed in the post about the same question just a bit further down the page entitled How to get rid of septoria leaf spot for good? you can't.

As Dan said the spores are in the air. Has little to do with where you bought your plants. It is the most common tomato plant disease worlwide so as often discussed here all you can do is try to prevent it by applying fungicides from Day 1 in the garden.

Dave

Please post pictures.

In order to daignose a problem, we must see what kind of leaf spots are present.

Some of the above guesses may be on target, but we'll not know for certain without images.