I have to say you're being a bit two-sided here. On one hand, you say you weren't abusing the system as if you were just using it as intended. On the other hand, you say you used the "trick" to buy tens of thousands of dollars worth of cards at a discount, well knowing this is not the way the site was meant to work because you call it a trick.
Aside from that... Can you share what volume you did thru what time periods? You only said 5-digits, which could be $10,000 (not much at all) all the way up to $99,000 (absurd amount). Concealing the amounts doesn't really help anyone pin down the levels that get one blocked. I don't believe their reasoning is arbitrary. Surely the person who blocked you is in one department and the rep you're talking to has no clue who that person is, nor why they blocked you. This may be by design. If they tell you the reason, you can plan to circumvent it. Right? Target would not want you to know the exact reason, else people would just try to skate right up to that line but not cross it. I don't believe they are obligated to give an explanation.
Can you just buy your remaining cards in store? You'll still get the 5% off if you pay w your Red Card.
Well, that's a bummer.

Options are drying up.