I agree that the pony is unsuitable for the job.
Ponies are thinkers, more so than horses. She has learned that she can get away with such a mundane boring job and will continue to do so. If you think she will be fooled by a sandbag then think again. She will soon take advantage of whoever is leading her because they do not know what they are doing.
As for pain issues - it could well be, but more likely it is just cunning.
All these ponies would be happier in their work if they were taken out for a good happy ride a couple of times a week ridden by kids that can ride.
As for stopping her I would have a rider on her - one that can ride. They would sit and act like a novice and when she kicked at the side walker she would get a darn good wallop from both the side leader and the rider.
Many will not like this. I do not care kicking is dangerous and unless she gets a short sharp lesson she will continue to do it.