You can lock forecasts and actuals on individual assignments for updating by using the Alocked field, which you can add to a custom view, or have your administrator include it in a standard view.
Fields related to assignments locks are:
Alocked - Setting this to "Y" locks the assignment from forecast or actual updates.
ALockedBy - Shows who last locked the assignment.
ALockedOn - Shows the date the assignments was last locked.
You can see the Alocked field in a view in the screenshot below:
For more on the types of data locks available in ResourceFirst, see Data Locking in ResourceFirst.