As a countryside manager myself I wouldn't advocate the removal of a burr from a live healthy tree. Any invasive works could compromise the trees health and lead to long term problems ultimately resulting in the trees removal. As Robin suggests, tree works are occuring legitimately everyday and subsquently burrs and the like appear as a useless byproduct so there is little need to harvest from an already healthy tree.
There are a number of reasons why trees produce burrs and the way they are formed, the only one that would be reasonably easy to remove but would still cause unwanted damage would be those that appear through epicormic growth, these are anchored near the surface and are a result of the trees own physiology and genetic make up. Other burrs are more likely to be caused by fungal infection, previous trauma or damage or by gall wasps, the same family of insect that causes oak apples. These insects can cause the galls by a number of means, one by exuding their own enzyms to cause the tree to mutate or even implanting fungal spores which have the same effect.
aaaaaaanyway that is far more information than you asked for and I'm showing a slightly nerdy side! Suffice to say that burrs are made up of a mass of chaotic grain that is impossible to remove without damaging a tree.
Leo
