IDK Missy, since I haven't been able to find out anymore. The way I read it so far, you should figure all your remaining birds are in fact carriers already and your property already "contaminated". It doesn't seem feasible that you could clean up or do anything about it when they say it can blow in from even miles away. Your only fear in regards to Marek's and bringing in new birds is to the new birds themselves. If they are younger than 6 mnths and were not vaccinated, they are at risk, no threat to your existing stock. Since your new chicks will be vaccinated already and so long as you continue that practice for any new chicks, you should be fine in regards to Marek.
I don't know about non-vaccinated birds over 6mnths. You can't vaccinate them I guess, but everything just says it's so common and contagious that all birds over 6mnths you should assume have already been exposed. I don't like that. Does anyone know more or where to get more info? How old was your DS's roo?