Hello Paula
To help us answer this in the most accurate way for the situation,what is the reason for the write off?Have they issued you a credit note/refund or is it the case it is something you will not be paying?
You can either
Create a credit note and use that to pay of f the bill for that amount
or
create a journal entry and use to 'pay off' the supplier balance using the creditors and supplier name on the 1st line,the 2nd line would be the account you are putting the write off to. Do you have a bad debt account set up for your debtors? Are you wanting this to show on your balance sheet?
It all depends on how you want it to show,apologies for the questions but we can only give general advice as we are not accountants
Emma