yes it's not very hard to jam, there are many jammers available online, a top result in a quick search was ironically from a site called jammerfromchina.com...[1]
Also, with an alfa wireless card and python (scapy module) it can be done pretty easily [2].
Can light be used to send messages when wi-fi is jammed? I mean modulating the torchlight and using cams. I realize this would be very low bandwidth though...
Well, radio waves consist of light, albeit outside of the visible spectrum. It is certainly possible to transmit signals using visible light (see Li-Fi), although visible light transmissions could be jammed using the same techniques.
Might not be ideal for covert communication though, since you could see someone broadcasting without any special equipment.
[1] http://www.jammerfromchina.com/categories/WiFi%7B47%7DBlueto... [2] https://github.com/DanMcInerney/wifijammer