Virtual network embedding (VNE) was subject to extensive research which lead to the emergence of a large number of efficient VNE algorithms. When virtual networks (VNs) arrive and depart over time, the substrate network can easily drift into an inefficient configuration, where resources are increasingly fragmented causing a VN request rejection although cumulatively, there are enough available resources. The ability to reallocate running VNs clearly leads to a better resource utilization. In this paper, we propose " Garbage Collector " (GC), a novel network control program for dynamic and online resource management in virtualized SDN-enabled substrates. GC efficiently addresses the fragmentation problem by performing selective migration. Simulations show that GC clearly improves acceptance ratio of VNE algorithms. They also reveal that, it outperforms some existing works from the literature by increasing the VN acceptance ratio by more than 10%.