The way I see it you don't have to use the feedback system to "warn" other sellers. It can't work.

The best way to achieve this is to open/close a UPI case. This action records a strike on the buyer's acccount.

If other sellers have their correct buyer requirements in place then once a buyer gets 2 or more of these strikes they cannot bid or BIN from any seller who has blocks in place.

If it's just recorded in a buyer feedback comment then there is nothing to stop that buyer from bidding at the last minute (or buying BIN) before a seller has time to look up their feedback comments. So it cannot warn other sellers.

So don't waste your time with such feedback and risk a slap for a policy breach yourself.

Put the energy into the existing system which works well.

We simply don't bother to leave any feedback for non-payers. All this does is give them one more positive score on their account regardless of what the comment says.