How do I block someone from bidding

I've had quite a few buyers not following through with payment.  I always go through the process of Non-Payment so that they get a strike, but I would also like to create a list of non-payers so that they can't buy from me in the future

If you haven't already done so change your buyer settings to auto block buyers who have two or more non payment strikes within a 12 month period. This will weed out most of the serial non payers.

But you can also add their user IDs to your blocked buyer/bidder list. This means they cannot buy from you regardless.

An even better option would be to turn your listings into BIN with Immediate Payment required. That will solve 100% of your non-payment issues and save you the hassle of opening and dealing with cases.