A Center Controller reported a UAS operated at the ceiling of the Special Use Airspace requiring the Controller to issue traffic calls to enroute traffic overflying airspace.

Narrative

Airspace R2501ABCDE active from surface to FL280. Aircraft X is orbiting within the confines of R2501 at FL280. Target continuously merges with incoming LAX arrival traffic so traffic calls have to be made for hours while the aircraft is in there. The paperwork for this drone states that he will be at FL210 or FL220 but that is not the case. The information for this flight should be obtainable through the Military Occupational Specialty (MOS) desk. Aircraft X was there on Day 0 and Day 1.Aircraft should not be allowed to be at the top of the airspace. Participants should be restricted to flying no higher that 27;500 in this case as to avoid the numerous traffic calls and other safety factors. Because this arrival is part of a descend via procedure; many aircraft have deviated from their clearance to stop their descent at FL290 and have required controller intervention to stop the aircraft from entering the restricted airspace. With a drone operating at the top of the airspace there is no buffer to prevent an accident should any pilot deviate from their clearance.