If it were me it would be removed and replaced by a resistant selection, or, if it appeared necessary an entirely different kind of shrub. Note that, as with roses some varieties promoted as resistant by commercial sources do not fulfill the promise when sent out to retail sales yards - at least in my area, where the regional environment favors scab and similar conditions. Don't take this as an indication of hopelessness, many different kinds have been selected over the years. There may be a clean and hardy one you can find on the general market at this time.