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Take 'em with you to the grocery store or BestBuy and place them on the pad that says "do not place credit cards here or they will be demagnetized" I have no idea if it will work or not, but it's a free attempt.
I have my doubts about that Harbor Freight tool to demagnetize anything. I use a Radio Shack 44-233A VCR degausser, it does a through job. I can even demagnetize track and switches with it, very useful to prevent magnatraction engines from picking up stuff stuck to the track.
Might be overkill for what your looking for, but we have one of these in our shop and it works. I have never had much luck using the one that harbor freight sells.