Lens getting fungus again and again

The best way to inhibit the growth of fungi in lenses that have already been infested and cleaned is to periodically expose the internal lens elements to UV radiation. This can be as simple as regularly exposing them to sunlight for a few hours, or as complicated as buying a dedicated UV light to shine through them.

Enough UV light in sunlight reaches the Earth's surface on sunny days to affect mold spores and inhibit them from growth. Nothing that wouldn't also destroy your lens, or at least make it so radioactive that you couldn't safely use it, ever actually kills mold spores. UV radiation doesn't kill spores, it just causes them to remain dormant for a while. Thus you need to repeat exposing the lens to UV periodically. Just be sure that the lens in question is not left unattended with the sunlight striking the lens elements in such a way that the sunlight is focused on something flammable at the other end of the lens from the sun. That's a good way to start a fire!

If you decide on a dedicated UV light source, be sure to follow all warnings carefully. Excessive exposure to UV light can cause all sorts of health problems, from degenerative vision up to and including permanent blindness if too much is allowed to reach your retinas to skin cancer if your skin is exposed to it too much.