I use a very small "alan wrench" - it is small enough to fit into the mouth-piece hole but strong enough to pry those stubborn ones off without bending.
I'm using a tiny allen wrench too...started with the OP's seam-ripper when I first started refilling, but the one I had was so cheap, it bent so many times while working around in a circle to carefully pry the cap off, the sharp end eventually just busted off
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