You can set up a VPN and connect directly to their network. You can put one of their computers into the DMZ and expose all its open ports to the internet. You c

You can set up a VPN and connect directly to their network. You can put one of their computers into the DMZ and expose all its open ports to the internet. You could engage in denial of service by blocking their devices or messing with the WAN configuration. You could set their network to use a malicious DNS server and direct them to hosts that you control.