Low voltage "jamming" for practical jokes

[AndyBeez]

That circuit is a standard tone for tuning the IF cores on old ferrite and wax FM radios, or for testing test equipment

If you want to prank WIFI, a cost effective method is to Spoof Jam - which is swamping the known WiFi channel with a stronger signal, often from a 1000mW industrial WiFi repeater. Then create a wireless interface that emits the same SSID, BSSID/MAC and security protocol. Client devices will be somewhat confused.
Practical? Yes. Funny? Maybe. Legal? Unlikely. Gets your ass kicked by your mates when they discover the evil genius behind the prank? 

[MathWizard]

What if you put the phone in the microwave, I bet that's on youtube.

When I was younger, I always wanted to make a TV remote control jammer/scrambler, IDK if they still work or if basic remote IR signals have evolved since the 90's.

[p.larner]

years ago i noticed everytime i used my microwave oven the wifi dropped connection,on investigation the oven had a hole rotted out under the platter,it went in the bin.