There's no way of doing it automatically. If you spot a candidate then you'll have to do a manual entry. If they've already bid and the auction is still live, cancel their bid and then block.
If you haven't already done so, go to your buyer preferences and check that they are still set tight, especially to block people who have 2+ item strikes/4+ policy violations in the last 12 months.
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I bid, I win, I pay - you deliver