Open any ongoing project and navigate to the budget editing section.
Here, you'll see padlock icons on the right side of each budget item.
You can click the padlock on any item to lock it:

Locking a budget item
Another option is one of the Advanced Functions — the Lock All Activities option:

Lock all budget items
Saving afterward will lock all items where the padlock is closed.

Locked items in Time Tracking
When creating a Recurring Project, you have the option to set what happens after the project's period ends. If you choose to Lock it, all items of the project will be locked at the end of the period.

Setting item locks for a recurring project