Hi Renee,
You can terminate the contract by using Notice Reasons.
You can configure any notice reason as per your business requirement and then Give Notice to the particular contract.This in turn updates End Term date in Contract.
Once the contract gets End term date, cashflow of future line items if any gets stopped from that particular date.Then you can lock Contract Master data and account assignment.
Hope it helps you!
Regards,
Siddharth