You can use the deleteDynamicCluster.jacl script to delete dynamic clusters.
wsadmin -f deleteDynamicCluster.jacl node_group_name dynamic_cluster_nameIn place of node_group_name and dynamic_cluster_name, use the name of the node group that you want to disassociate with your deleted dynamic cluster. You might need to modify the wsadmin command to wsadmin.sh or wsadmin.bat, depending upon your operating environment.