This is the situation you find yourself in on walks, because your Cocker spaniel has a bad habit of barking at joggers. You know your dog and understand that it's just because seeing a stranger running towards him makes the dog feel anxious. The dog's natural reaction is then to bark at the jogger, to make him go away. This is unfortunate because to strangers it appears that the dog wants to do them harm, whereas you know nothing is further from the truth and he's just frightened. You've tried reassuring the dog and telling him there's no need to bark. Sadly, not only does this not seem to help, but joggers think you are praising and encouraging the unwanted behavior.
Nobody likes to be misunderstood, but it's even worse when it's the dog.
Most dogs will bark at a jogger because they either feel threatened as a stranger runs towards them, or because the dog becomes over-excited by the idea of a chase. Unfortunately, the fact that the jogger goes away accidentally rewards the dog, who then thinks that barking is a great strategy for getting rid of a threat.
Instead of getting frustrated with the dog, use obedience training so that the dog concentrates on you and is less likely to feel threatened by the jogger. This will also give the dog a greater feeling of security because he's aware you are in control of the situation.