Is this a Grey, or a Red, fox?
If it is a native Grey, then I'd be more interested in `workarounds' rather than elimination. If it is a Red then retirement from the target pool is the responsible method of control. Red Fox, like Zebra Mussels, Multifloral Rose and Amur Honeysuckle, are imported `invasive ornamentals'. Promotion and toleration of these species is nothing but trouble for native flora and fauna.
Not having to deal with Greys, I can't comment on their taste for chicken. The Reds are brazen and have attacked ours while we were no more than 10yds away (out of the tree line and grab a chook). Our neighbor lost all 13 of her SLW pullets in the space of 15min. to a pair of Reds (found the caches in the woodline between our properties a few days later - nothing but feathers).
As has been mentioned, electric fencing, trenching/skirting with fencing on ground outside of run will eliminate most problems.
However, all I can say to a certainty about free ranging (if one is not accepting of losses) is that armed/supervised ranging is useful. Preemptive trapping and retirement of vermin will not keep more preds from coming, but will reduce the overall frequency of predation (so one doesn't have to reach for the rifle as often).
Using scents to dissuade predators will work initially but, preds learn. Eventually, they associate the smell of urine/etc. with chicken and start salivating with the learned association of the two.