If it's the old 2.4ghz wifi, everybody already has a powerful one in their kitchen, if they just defeat the door interlocks.
Well yeah, and the added benefit is melting the candy bars in your pocket...
There are 3 ways to go about defeating wireless systems (not just wifi, rf in general) flood over ride, in other words you can put out so much noise the signal is lost in the mix (like the old tractor, probably had an old uhm.. not alternator.. Generator thing, sorry brain is still jello from my weekend)
The 2nd way is simple, burn the finals, if you know where the receiver is, get a moderately large source (high power) and get physically close to the receiver, point the (hopefully directional) antenna at the receiver antenna and fire it off, keep it running as long as possible. Only the very best of units will handle this, and those will require the case being opened and fused systems or gear bridged circuits replaced.
(this is how Germany attempted to take out british radar units on the land, but they tried crossing "open fields" which were in use as a deployment group departure point)
and lastly, the one near and dear to the hearts of all the IT guys who were around for it. Cisco systems in the oughties put out a meshed system that was "built for the cluttered environment of cities and large buildings" in other words, it was going to use intelligent channel switching. Which most the other companies already did. In order to make it more competitive, it would latch the signals of other "competing" units and then flood those channels, it would then "train" the surrounding "smart' managers to stay off the channels that the cisco unit wanted to use. Which was great, and it of course violates every countries laws about use of the RF spectrum. The US is pretty restrictive, but *ALL* countries have rules about aggressively attacking things, that is reserved for government entities. This system was quickly recalled and replaced, The problem that existed is that the latching/flooding system was not in the controller, it was an add-on circuit in the transceivers. So all those old cisco units out there have that circuit as a snap in... of course I am not suggesting anything.
SO rumour has it that the latch and scramble systems that appeared on the grey market around 2010 might in fact have been either knock off of the cisco circuit, or are in fact just ones pulled from the trash bin at Cisco....
Latch and scramble is the holy grail of how to mess up your neighbors, as a circuit goes it is not hard to design, as an implementation goes, it is a bloody nightmare to get them tuned in. the best use is to get badly tuned ones, pair them into groups and then just let them detonate the spectrum in their area. Add to this a directional antenna with a 2-3kw linear amplifier on it, pop it into the top of a truck with a tilt over camper shell and you have a mobile denial system.
:;looks around:: what? It is talked about in the industry. I never did it... well, not with civilian hardware.