These are the steps needed to write off an office balance with an unallocated receipt balance/nominal balance:
Open General Postings.
Create a BPN posting from the write off nominal code for the value of the balance.
Create a BRO posting to the matter for the same value.
For a nominal balance create a BRN to go against the nominal with the balance (you can also just do a journal between the 2 nominals).
These postings will contra on the bank reconciliation and will clear down the unallocated receipts balance too.
If the balance is due to a TOC posting then the TOC can be posted the opposite way and posted again on the ledger with the Disbursement box ticked. Then this can be written off through General Postings as a WO.