Okay, I was hoping somebody with more knowledge than me would chime in and answer your question. Alas, no one has, so I'll give it a try, but remember I'm probably not the best person to be answering these questions.
I'm pretty sure there are routers that have a physical WiFi switch, I just don't know which ones. All the routers that I've dealt with have a software switch to turn of WiFi. You have to log onto the router, find that setting, then toggle it on/off. The downside of this is that you'll need a hardwired (Ethernet cable) connection to the router to turn it back on.
An alternative solution would be to use a dedicated modem and separate routers. You would need a dedicated modem (one that doesn't have a built in router), a router without WiFi and a router with WiFi. You would connect the router without WiFi to the dedicated modem and then daisy-chain the router with WiFi to the first router. With this method, when you need WiFi you would turn on the router with WiFi, and when you don't need WiFi you would turn it off.
There are "sniffer" apps available, but I've never used them and can't really recommend one. From what I've heard, they are just some software that you run on your laptop.
I know this probably doesn't help a lot, but maybe by bumping this thread someone with more knowledge than me will chime in.