No need to get upset.

In the last few years, we've been led to believe that recognising any differences at all between people is somehow evil and discriminatory but in fact, looking at patterns and being able to make some generalisations is how people learn and how we all manage to get along in life.

Sure, not every new buyer (or seller) is trouble, but if you are finding the trend is heavily towards your zero feedback buyers giving you problems, you'd be mad not to act on it. Same with some countries. If sales to a particular country give trouble out of all proportion to their numbers, then there is  probably a problem with postage to that area, so only sensible to cut them out.

Yes, you'll probably miss out on some sales that would have gone fine, but on the other hand, it's not a bad trade off if it cuts out 90% of your problem sales.