They might get strikes on their account, which seller blocks can prevent them from buying from you, but I did see a buyer several years ago that had over 1,000 false positive feedbacks for non payment (even though false positives are a violation of ebay policy). Obviously ebay didn't care too much about them. Put blocks in place to stop the serial pests and you'll be fine. Anyone with 2 or more strikes in 12 months won't be able to buy from you.
