Yep, I think using a muzzle is perfectly acceptable to stop a really determined scavenger, especially where there are potential health risks involved. As @JES72 says, breaking the cycle is sometimes all that's needed. In the same way you'd clean your kitchen counters or close the door, the muzzle is simply a management tool to prevent self-rewarding. If the alternative is to have to keep your dog on lead, I would definitely prefer the muzzle approach, assuming the muzzle is introduced sensitively and accepted by the dog. Also assuming that the owner continues with training, if that's appropriate. I can imagine if I adopted an older dog who developed or had an existing ingrained problem that could damage their health, I'd probably just use the muzzle without trying to train the behaviour out. Pick your battles!