You can do something as simple as say "Turn on airplane mode" and that would disable data (and wifi too at the same time).
BUT... Bixby needs network connectivity to work. You can't even have Bixby give you the time of day without some sort of network connection.
So you'll have to turn data back on yourself. But at least by using airplane mode you can just swipe down to get to your notification panel and have Airplane mode there. Of course I guess you can do the same with data.
You can say, "Turn on/off wifi" no problem because you'll still have network connection for Bixby to work.
At least that's what I just tested unless someone else can get Bixby to work when data is disabled.