It's not that hard to achieve....
If you have a regged gun and you set the reg pressure deliberately too high, you need to wind up the pre-load on the hammer spring to compensate. If you choose a hammer spring length and spring rate that becomes coilbound on cocking, the owner can't increase the pre-load any further.
If all this has been done with the transfer port venturi wide open in the first place, short of changing the spring or stripping the gun down to set the reg pressure where it ought to be, you'm stuffed as they say down 'yer.
How did I find this out?