Data Cell Conditional Input rule ???

So you want data to become read-only on the *source*/originating scenario after you've copied data to another scenario?  Or you want the data to be read-only on the other scenario.

If you want the data to be read-only for forecast-type scenarios then you can do something like this:
Select Case api.FunctionType

Case Is = FinanceFunctionType.ConditionalInput

If api.Time.IsFirstPeriodInYear() _
AndAlso api.Scenario.GetScenarioType = ScenarioType.Forecast _
AndAlso api.Account.Text(api.Pov.Account.MemberId, 1).XFEqualsIgnoreCase("FC_Readonly") Then
Return ConditionalInputResultType.NoInput
End If

But if you want to lock certain intersections in the *originating*/source scenario after the data has been copied then you could have a system account (could be in a separate cube) which tracks whether a data copy has been performed or not, which would be updated using the DM job, for the originating time/scenario. Then the ConditionalInput rule could read that system account,
e.g.   A#[DataCopyCompleted]  , and if it contains a non-zero amount then consider certain intersections blocked on that scenario.  Then some administrator could reset it on request in exceptional cases.