Killing a starter with garlic may not be quite the same as raw garlic killing good bacteria in the gut. It doesn't take much to knock a starter out of kilter. For example, kefir starter won't work with coconut coconut doesn't bother the cultures once kefir is made.
I brew my own filmjolk and a starter just a couple weeks old will make it difficult to start the brew never mind contaminating the existing brew with garlic.
Corn meal scattered on the ground will prevent certain vegies from germinating as well as weeds. But once up corn meal is a great fertilizer. Think garlic works the same.