- Carry a battery-powered wifi jammer whenever you're in a public place
- Permanently disable the wifi chip
- Persuade the manufacturer to implement some real security in the device
- Wear a Faraday cage
1 would be technically easy to do, and you could probably keep the power low enough to avoid affecting nearby equipment, but appears to be illegal in the UK and USA.
2 is also easy in theory - the chip's antenna will make it more susceptible to damage by radio/microwaves than the rest of the equipment, so a few seconds in a microwave oven should destroy the wifi without affecting other functions. Probably not safe for a pacemaker, though - just in case it shortens the life of other components, not to mention that you might actually need the wifi connection for its intended purpose - Plus, it shouldn't be tried in situ!
3 is the best approach if it could be made to work, but I wouldn't stake my life on it in the short term.
Which leaves us with 4. A full suit of plate or chain mail armour, including metal boots, gauntlets and helmet should work if you added enough electrical connections between the various sections, but would be awkward to wash and wear. I suspect you could get away with a lot less - such as a jacket with a buried copper mesh - but RF shielding is a bit of a black art (to me at least) so I'd have to experiment.
I suppose a possible 5th approach would be to carry a wifi-enabled device that continually listens for other devices, detects when one of them is connecting to your pacemaker, and then jams the signal (in self-defence, your honour...).