I had the problem...
I had the same problem (consistent among most Yanmars) until I replaced the throttle cable with a higher quality Morse cable as part of normal preventive maintenance. Prior to that my simple solution was to use a light cord with a loop on the end. The loop went over the throttle handle and the other end was secured to the binnicle guard with a clove hitch. When I set my throttle, I adjusted the clove hitch to prevent the creeping back.Basically, the reason for the creep (at least on my Yanmar 3HM35F) is that the throttle lever (on the engine, which the cable connects to) is spring loaded to return to the lower throttle setting. Then it eventually creeps back to the lower RPM's.