These cankers are a result of branches that broke sometime earlier, probably at least a year ago. Bacteria and fungi colonize the exposed wood and cause decay in cases like this. Branches and entire trees can become unstable as the decay progresses.
A canker like this cannot be removed. Anyone promising to do that is selling something they cannot deliver.
Promoting the overall health of the tree by reducing drought stress and competition for soil resources is the best way to handle this. You may need to consult a, ISA certified arborist who can give you an appraisal of the tree's health and possibility of it falling. You can find a qualified outfit here:
http://www.isa-arbor.com/findanarborist/arboristsearch.aspx