Lameness like Reovirus infection in young broiler chicks , spreading rapidly in the flock,but ruled out Reo,inviting suggessionsn

Why REO was ruled out ? Did you checked for Mycoplasma synoviae ?

Did you try with hydrogen peroxide disinfection (no birds)?

What other symptoms are seen besides lameness and what is the mortality %?

Such incidences may occur due to non REO vaccinated parent flock chickens , Staph., MS infections or due to certain deficiencies and sometimes mycotoxins also play a role. However, they should respond to indicated antibiotics and highly bio-available Calcium and other nutrient source, if it is not due to REO. You may need to recheck all the above to find out the cause and treatment as well.