Without the receipt, it was hard to go back to the shop and ask to get it taken off there, so we had to do it manually instead.
Twenty minutes using two pairs of pliers and a screwdriver did the job. The balloon (and a plastic bag and latext gloves) served as insurance in case the seal broke and poured out deep-penetration ink. It probably wouldn't have come out anyway as we put the jumper in the freezer overnight to solidify it, but we weren't taking any chances.
Next time, I'll do it without my wife overseeing me. That way, it will take much less time and I'll get to see what the ink in those tags looks like.