Hey chap!
It's not possible to totally eliminate all ich parasites. You would have to sterilize all your substrate, LR etc... I don't think you would like to do it!
However, it is possible to control the level of parasites in our tanks and bring it to the level that is found in healthy tank. This is achieved not only by medication, but more importantly by proper feeding (vit. C/garlic enhancers), reasonable application of water additives (not to introduce to much stress) and general proper water maintenance that keeps our fish/inverts immune to the parasitic infection.
Biologically speaking, if you detected ich infection:
1. the general conditions in your tank are bad. This affects your fish that can not generate popper immune response to the parasite that is normally present in their natural environment.
2. natural enemies of the ich parasite, that normally compete with it for the primary food source (I mean non-parasitic small protozoa) are not present or have been wiped out recently, moving your system out of microbiological balance.
I've red many articles about the ich, talked with some marine biologist and many of them consider ich as a stress related disorder!. In other words, most of them claim the cause of disease is stress and one of the symptoms is the parasitic infection! | 
