To stop the fungus attack, put the lens in a dry place. If the fungus growth is not too advanced, you can have your lens cleaned at a professional service center. As stated by Zewrak, don't attempt to repair it yourself. If the fungus has already started etching the glass, there is nothing that you can do to restore the lens to its initial condition.
To prevent this from happening, you can buy a dedicated dry cabinet, like this one:
If you don't have the funds, putting a small light bulb inside a cabinet as suggested by Tom will do the job. It is better to supply half the nominal voltage of the light bulb (like supplying 12v to a 24v bulb) in order to make it emit more infrared and less visible light. A power wire wound resistor will also do the job.
Cheers!
Abbazz