Welcome to unlicensed frequencies. You can also buy any number of legitimate narrowband continuous transmitters (e.g. an analog video transmitter) and effectively destroy one of the available channels.
I've got a cheap Chinese 2.4GHz analog video transmitter in a box somewhere which is _way_ overpowered (600mW from memory, instead of the legal max of 25mW) it completely knocks out all my 2.4GHz wifi when I switch it on (and I strongly suspect all my neighbour's wifi as well). I've got much a less overpowered 5GHz which my 5GHz wifi doesn't seem to care about.