Do I understand this correctly? The vCenter Server has been decommissioned and now you get the message that the ESXi host is managed by a vCenter Server when you connect to it using the vSpehere client?
You may connect the ESXi host to another vCenter Server which would reconfigure the vCenter Server agent. If you want to configure the ESXi host as a stand alone host you need to remove the vCenter Server agent. Either connect/disconnect the host to another vCenter Server instance or remove the agent manually. For details see http://kb.vmware.com/kb/1003714