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To prevent the tablet from connecting to WiFi...
Try turning on Airplane mode (airplane mode can be over-ridden by turning WiFi back on for planes that offer wifi).
Try turning off WiFi
Try putting it in an aluminized Mylar bag.
Try turning off the tablet.
Try hitting the tablet with a hammer or large stone until it no longer connects.
Short the pins of the antenna connector to itself? Find the shielded RF section, remove the shield and leave it off. Maybe muck with what is underneath a bit. Or send 10V down the antenna port and hope it fries it without frying anything else?
There are legitimate reasons for needing to disable the wifi portion of a tablet.
At the naval shipyard near here, if you bring a cell phone into the controlled area, the camera must be removed and the hole filled with epoxy for security reasons. There is a local industry of people who preform the service.
There are areas where no cell phones or any type of transmitter is allowed. A tablet may be useful in those areas, but it would be required the radio be permanently disabled.
Can't say if this is the OP's need, but there are legit reasons for doing so.