I don't think you can ever kill all of the root knot nematodes. Your best bet is to find grafted roses, preferably grafted onto Fortuniana. You could also try growing roses in large containers, but I think the nematodes probably would find their way into those also in time.
There are some places in Florida where the root knot nematodes are not as big a problem as in others, but I don't know where they are. Ask someone at a local nursery whether your location is full of nematodes.
