posBoss will clear the account history when the account is paid off in full.
If the account is only partially paid, the history will remain in position until it is paid in full.
If the customer is not paying off the account in full, you could look at setting up a monthly account for the customer and name the account for the month such as "customer name - July 2017". This way the customer will need to settle up at the end of the month and you can archive the old account when no longer needed.
Still unable to remove the history
One scenario that can cause this is is due to placing discounts on an account that tries to split 1 cent.
An example of this would be placing an assignment of $38.67 onto an account which has a 50% discount. So on payment, it tries to split 7c and then has a bit of an issue.
You can fix it up by paying off the account, then placing three cents onto the account. then paying it off again. You could add a small item and discount it by all but the three cents. This will clear up the discrepancy and remove the history.
Alternatively, add a discount on their next order that brings the total to an even number.
Longer term, try not to give discounts that split an uneven number if possible.