First remove the allocated floating IP . use command floatingip-delete IP after that delete PORT using port-delete portid .After deleting port you can be able to delete subnet easily

Hello,

Looks like you are having an orphan port.

neutron port-list

Then remove the port using neutron port-delete <port id>

After that remove the subnet using

neutron subnet-delete <subnet id>

Your issue is related to this review https://review.openstack.org/#/c/127357/

Delete Subnet
This operation remove a subnet from a Neutron network. The operation will fail if IPs from the subnet being removed are still allocated.
409 Conflict 

Delete Network
This operation will remove a Neutron network, and all its associated subnets, provided that no port is currently configured on the network. If ports are still configured on the network being removed, a 409 error will be returned.

Hi this is the simple and confusing situation but you have to follow some steps and hope it will work :

neutron router-list

neutron router-show <router-id>

neutron router-gateway-clear <router-id>

Check for the attached ports to the router. If there are attached ports, clear them first.

neutron router-port-list <router-id>

neutron router-interface-delete <router-id> <subnet-id>

neutron router-delete <router-id>

neutron subnet-list

neutron subnet-show <subnet-id>

neutron subnet-delete <subnet-id>