There's a broad spectrum of activity that could fall under what you describe. Consider the following situations:

1. A $1000 purchase just barely pushed you over the 250k LP threshold. After that posts to your AA account, you later refund the $1000 purchase but in the same billing cycle make $1200 in other purchases. The bank will report a net gain of $200 of spend for that month and AA won't even know there was a refund of the purchase that happened to push you over the 250k threshold.

2. Similar purchase to above, but on the statement that you refund it, you only make $800 in other purchases. In my experience, what happens is that the bank will carry forward a $200 negative balance as far as purchases eligible for AA credit goes and report $0 qualifying spend to AA that month. Then, your next $200 of future spend on the credit card will eat away at that negative balance until you start earning LP again. AA still doesn't know the refund happened.

3. You continue to use the card for a few months with several thousand dollars of spend, then refund the $1000 purchase that put you over the LP threshold and close the credit card. I think it unlikely anything bad is going to happen.

4. You're $1278 short of the 250k threshold. You make a $1000 merchandise purchase and a $278 gift card purchase, putting you exactly at the 250k LP threshold. The day after these post, you refund the $1000 purchase and close the credit card. I don't know if the $1000 spend deficit gets reported to AA to claw it back. I don't know if in the event that happens, whether or not the SWU's would automatically be taken back. There is the danger that Antarius mentioned above of this triggering an audit. If a human were to look at an account with this pattern of activity, it would be obvious that the person was trying to game the system, and they may conclude that the $1000 purchase was not legitimate and take back the SWUs.

I don't think anyone can tell exactly what will happen. I think that the likely outcome, especially if human eyes look at it, is at least somewhat related to the totality of the situation and what it appears the person was or was not trying to do, and not solely on whether or not the purchase that pushed them over the threshold was refunded.