Ah, but that assumes they know what you're doing. Microwaves have no qualms about passing through plastics, do they? Just don your best suit, carry an attaché case like an aspiring FleetStreeter and look interestedly at the ad under the snooping device. I don't think it takes 1500 W to bring it down - nor do I see how you'd achieve that power output from a discarded household magnetron - but a few hundred watts at close range will surely make the things' 'ears' ring.

If microwaves are not your thing you could try your hand at an EMP projector, even though it might be harder to make sure only the targeted device kicks the bucket.