Hi GLawrence,
1. No, if payments on account have been calculated you cannot remove them from your Self Assessment tax return.
2. If you think your balance will be less for the next tax year you can make a request to reduce your payments on account.
You can do this on your tax return or by submitting a SA303:
Self Assessment: claim to reduce payments on account (SA303)
Once updated the payments on account will be updated on your statement.
As close to the payment due date if you know the payments on account will be reduced you can make the payment from the new amount.
Thank you.