You can lock down an entire budget so that no more changed can be made.
This can only be performed by users with permission to access the closed status on the contract.
To lock down a Contract
1. Click respond on the Contract you want to lock down.
2. Change the status to the Closed Status type. Usually this is called "Executed" but may be different on your project depending on how it has been configured.
3. 
Contract line items
Adding and Editing line items within a contract is only possible whilst the contract is in the open status. This include formatting such as indenting.