It has to be something else like a duplicate IP address or something. I can't see how a light could ever use up even say a 5mbps internet connection. Its not like it has a hard drive where it can transfer files. In most cases the lights listen and take tiny little commands from some app. It not like a camera that would stream data constantly.
Does it even use the internet. I would not really want those type of devices exposed.
You can only really limit traffic going out to the internet not say between the wifi and the lan or other wifi. Your router would have to have QoS features. If it has firewall I guess you could try to completely block it. Still I bet is something else and not a traffic issue.