What is an OFAC "prohibited transaction"?

Question

so obvious first step: the official OFAC F.A.Q. (i.e. of the U.S. Office of Foreign Assets Control in the U.S. Treasury Department):

3. What does one mean by the term "prohibited transactions"?

Prohibited transactions are [....] transactions [....] in which U.S. persons may not engage [....]

Great! So "prohibited transactions" are "transactions" which are "prohibited". Now I just need a few small clarifications:

• Clearly it is allowed to receive funds from transactions initiated by designated entities, since celebrities have received over 100 USD (0.1 ETH as of 2023-07-07) without concequences. (unless OFAC regulations are selectively enforced for discriminatory purposes)
• How about if my online store initiates a financial transaction to receive funds from an an OFAC designated individual and then before completing the store transaction (as distinct from the financial transaction) realize they are on the SDN list. Am I fine so long as I do not refund the transaction?
• Could I refund the original debit/credit card transaction (i.e. the refund is performed without creating a second transaction, i.e. the original transaction ends in a refunded state instead of in a captured state)?
• Could I refund the original transaction with a separate transaction? (I would assume that it is not allowed.)
• Are OFAC regulations publicly available or are they private internal regulations that are themselves only loosely and indirectly limited in scope by laws and the executive branch (and thus without regard to constitutional limits like a rouge the C.I.A. many 3-5 letter government agencies, since the constitution now has no more effective value than toilet paper apparently.)?