Some were allegedly found with their hands tied and stripped of their clothes, spokeswoman Ravina Shamdasani said
The 31-year-old ended the fourth day with a flourish by sinking birdies in the final two holes to go 20-under for the tournament and finish ahead of his fellow overnight leader Rahm who managed 70.
Victory marked a superb turnaround for Willett, who had slipped to 462nd in the world rankings at one point after missing the cut at this tournament last season.
"It doesn't get much better does it? An emotional week. It was a good battle out there today with myself more than anyone else... it was nice to come through the other side," Willett told reporters.
During his slump since winning the Masters title at Augusta in 2016, Willett replaced his coach Pete Cowen with Sean Foley and also changed his caddie in a bid to start firing again.
"I was able to jump in there and I had the correct people around me. All the work that we've put in has come forward," Willett, who returned to form by winning the DP World Tour Championship title in Dubai last year, added.
"I've had an undying want to get back... I was willing to change whatever had to be changed and that's pretty hard to do... to jump full throttle into something you're not sure if it's going to work out or not."
Willett began with birdies in the second and third holes and had two further gains before a blip on the 11th where he settled for a bogey. But two more birdies towards the end made it six for the day as he sealed a memorable victory.
South Africa's Christiaan Bezuidenhout was third at 16-under while Americans Billy Horschel and Patrick Reed were tied for fourth. Justin Rose finished eighth ahead of Rory McIlroy and Andrew Johnston.
Some were allegedly found with their hands tied and stripped of their clothes, spokeswoman Ravina Shamdasani said
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