I don't know about this card specifically, but with Amexes in other countries and other cards, if the points are already transferred to the loyalty scheme, your card account just has negative points after a refund posts.

Future spending will earn points as normal but they will go towards paying off the negative points first. Only when the debt is paid off will points be transferred to your loyalty scheme.

You may be able to close the card account with a negative points balance meaning that you get to keep the free points, but if you keep doing this the card provider may not be happy with you.