do you mean the metal one on the outside bottom of the reg? the only way i can think is try to cut a straight line into it and use a flatblade screwdriver to try and remove it.
dont cut the body of the reg though as i would hate to think what would happen if you put a weakspot into the HP stage of that reg.
Has the same problem with a DS4, which wouldn't shift. Having boiled the bastard in 50% vinegar for a period, the hex recess rounded off at about 1.5 Nm more than the suggested torque for doing it up. I cut a slot across it, then bent two screwdrivers and broke a multi-tool blade trying to turn it. (You may have guessed that I don't like to lose!)
Ultimately, I cut a broader slot across it and put a wood bit (22mm, IIRC) across the slot as a lever, then tapped the other end of the bit with a rubber mallet till it turned.
Mission accomplished! Albeit with a bent wood bit also!
Despite the above abuse, the reg worked fine once serviced. I did, however, rotate it off my right post after I slashed my hand on the edge of the slot while doing a shut-down. I've now poured some molten wax into the slot as a short-term measure, though will need to replace that with something more permanent unless I can source a replacement part.