Abstract: Techniques for defining and enforcing a set of rules that regulate network activity on a virtual network within a provider are described. An activity rule that regulates network activity on a virtual network within a provider network is received, the virtual network shared by a first customer of the provider network to allow other customers of the provider network to connect resource instances to the virtual network. Network activity data is received from a monitoring agent within the provider network, the network activity data relating to an activity of a first resource instance of a first other customer on the virtual network. The activity of the first resource instance is determined to violate the activity rule. The first resource instance is isolated from at least a portion of the virtual network.