If you have posted cash on account, which nets to zero, unless you have an unpaid invoice on the account there is no way of clearing off the transactions.
This is especially troublesome if the bank has been reconsiled and or VAT submitted.
The only way I have found to do this is to create a 1p invoice, allocate the cash, then reset the invoice to FOC.
Oh for sage.... where you could go to the bank and net transactions off to zero to your hearts content