Sorry to hear it. I have had a few cases of Marek's but the rest are fine, though they may be carriers, so out of many hundreds two or three cases isn't terrible. Not that the individual case are happy things to deal with of course.
As Nambroth noted, biosecurity won't save you here, and...
Have you tried Hypericum/St John's Wort? Can be used in homeopathic or pill form. It's used in humans to treat neurological diseases and disorders and some people on this forum reckon they've successfully treated their birds with Mareks' using this.
Best wishes.