@John2657 Best way is to have the ability to change the code for each guest and only give it out just before check-in. Don't put it in the area that is sent to guests as soon as their booking is confirmed, (especially considering that guest verification is not always reliable) they can always cancel after they have ransacked your place (not being dramatic, it was reported in some newspapers as a known scam issued by police departments in Florida). You can write instead that you will send it to them before arrival.
Maybe other hosts have better ideas of how to deal with this.