Is shunning effective. Here's some things to consider.
66.6% of DFed people never return.
From my observation (and although I have no evidence to it)I would say that the frequency of DFs is increasing.
Some DFed ones then take their issues to the internet which in turn creates further havoc when it's members and those thinking of joining the group read it. Hence the 2014 increase in most western countries is 1%. The world is growing at a rate of 1.3% so they weren't even able to retain the rate at which the world is growing.
I personally think shunning just sweeps issues under the carpet. It antagonize and makes the shunned one worse and more determined than ever that they will never return.