Here's what worked for me. Look under the resources for the custom location and delete the logical networking object.
Can't delete a logical network
When trying to delete a logical network, I get the following error;
There was some error while deleting logical network: VLAN_3508_3_5_DPT_Jumpbox_Area. 200: admission webhook "deletionvalidationwebhook.infrastructure.azstackhci.microsoft.com" denied the request: Deletion of default/vlan-3508-3-5-dpt-jumpbox-area failed. It is in use by other resources. Resource type: *v1alpha1.AzStackHCILogicalNetwork
There are no VMs built or running. There are no NIC resources (visible by showing hidden resources). Have also tried the CLI to remove. Any way to find out what is using the network (which I doubt)?