The only way out of an avian virus is to start over with new chickens, preferably baby chicks obtained from a certified hatchery, and disinfecting the premises and waiting an interval between the cleaning and getting new chicks. With Marek's, the recommended "rest" is one year minimum, even with disinfection. With lymphoid leucosis, it's about a month after disinfection.
This is one reason it's important to know if you are dealing with a virus and which one. Marek's lasts much, much longer in the environment, while leucosis lasts just weeks.