I have never had a problem with a book I sent until yesterday.

One time someone sent me a book meant for someone else--I just wrote her, mailed it back to her, and she refunded me my postage on PayPal and sent me the right book.  No problem.

I gave her her credit back, but if she complained once, I would prefer not to deal with her in the future (on either end.)

Is there a way I can block her now that I've given her her credit back?

No, you cannot block another swapper. All you can do is keep a note of the nickname so you can cancel the transaction if one ever happens again. I think it'd be rare to have another transaction with the same member.

I'm not sure why the book you sent was marked RWAP. If the book you sent was unpostable then you did the correct thing in returning the credit. If the book you sent was postable and the requestor had no requestor conditions, then you did not need to return the credit.

FIFO is not set up to block swappers at this time.  Fortunately, PBS membership is very large and active, you are unlikey to run across this member again.  Otherwise your only option is Rebecca's suggestion to note the nickname in the unlikely event you are offered another transaction from this member.

Your OP doesn't make much sense.  Did someone send you the wrong book and then complained because they sent you a credit and the correct book? Or did someone complain about a book you sent and what was the complaint?  If it's the first thing and they sent you the correct book and refunded you through Paypal,  then really, you don't need a credit as well unless you sent the wrong book to someone else.  Or did you send the wrong book back w/o asking her first?  I know, in this situation, if I was the sender-I wouldn't want to pay to get the wrong book back. It wouldn't be worth it.  I would just telll you to donate it or post or whatever you wanted.  (I would give a credit if it was supposed to go to someone else so you could mail it to them). 

I think it'd be rare to have another transaction with the same member.

It's rare, but it does happen.  My last 2 WL books came from the same member.  We must have very similar reading tastes.

Your OP doesn't make much sense

Mary, the way I read it is that she's talking about two different instances, one happened yesterday -- that's the one she has resolved but would prefer not to deal with the member again.  The other was a while ago... "one time."

I just keep a running list of members I've had problem swaps with-- name, nickname, and address. I keep it handy next to my computer.

Wow.  I only wish PBS had a blocking feature.  It would be so helpful for some users!