If wifi camera is 802.11x kind of camera it is really easy to get those jammed with wifi jammer and if it's 2,4 GHz and not 802.11x kind of camera then it's easy to watch video from those cameras example with Wireless Camera Hunter:
http://blog.petrilopia.net/security-2/wars...-camera-hunter/
Some of those cameras use same frequency as baby monitors so it might be possible to send your own video with baby monitor camera to those receivers... (Sorry my bad english... I haven't got enought coffee).