Servo malfunction possibility?   

Slide Valve Seizure / Actuator Malfunction (Bell).  More or less the same thing.  Something, usually FOD of some kind in the hydraulic fluid, jams the port open and fluid continues to flow, pushing the servo/actuator to full travel.  As I recall, it's like 400 lbs of pressure in the B407.  Unless you can curl 400 lbs, you're not stopping it.  All you can do it get the hydraulics off ASAP! 

I had a slide valve seizure in an AS350 once.  It was slow moving, but nothing I could do was stopping the cyclic from slowly moving right and forward.  Hydraulics off and landed ASAP.  Explained what had happened and the engineers drained and flushed the entire system, then replaced the fluid and filter.  I flew that ship for another year and it never repeated.