Hi,
You could try setting the "Ratio Weight" of the pool member to zero. This would prevent the member from receiving future requests. Not sure about existing sessions connected to the same specified server being dropped, but I would think they should remain intact since the ratio weight affects incoming requests, not requests that have already been serviced. Unfortunately, I'm not in a position to test this in the lab right now.
Check the following link for more info: http://blogs.vmware.com/vsphere/2013/01/vcloud-org-load-balancing.html