Maybe you educated him or her not to come around your place anymore. Been my experience once they get a Chicken they will be back for more, they just get sneakier.

6 shot is unlikely to do much damage at 40 yards IMO, most of the pellets won't make it through the fur. And butt shots are the least likely to put one down anyway.

I've tried various rounds 00 Buck to 4 shot, not all but many. The biggest disappointment was 00 Buck, I've yet to kill a Fox DRT with 00 Buck. I don't know if it is operator error (me) or what it is, I've just had poor results.

2 3/4, 2 shot, semi magnum (Rottweil) seems to work out the best for me. Pretty much a Goose round, but seems to plant Fox rather well, out to around 35 yards. I often get a shot at a Hare when Fox hunting, 2 shot works for them also. Hare here can get up to 15 pounds, we joke they are sometimes bigger than the Deer.

Maybe BB or 4 Buck might also work out, haven't tried them yet on Fox.

I average around 20 Fox a year here. But those suckers can be sneaky, I hunted one for two years that was depleting a guys Duck flock (most of a hundred Ducks). I finally ran over him one morning at 2 AM with my Jeep by accident. His luck finally ran out.

I pop the majority with my .308 out to around 350 yards. Most at the end of my Deer hunt in the morning, just before I pack up to go home. I sometimes scare one up (or my Dog does) when I'm Rabbit, Hare or Bird hunting.

I've also had pretty good luck live trapping them in places (like the Cemetery). Rotten canned Cat food seems work well. It can get ugly when a Fox starts looking for a meal in the Cemetery.