Security Question - can we block users from submitting data in excel? We want to prevent a user from submitting planning data on certain UD members

From Excel side, I'd suggest using Macro code to control the submission members in use at each data point. This way, you can pinpoint the specific UD member where submission is allowed. 

Like Eric suggested, you can also use "No Input" in Biz Rule - Finance - Conditional Input, don't forget to assign the rule to the cube in question.

If (Not scenarioType.XFEqualsIgnoreCase("Actual")) And (Not scenarioType.XFEqualsIgnoreCase("Budget")) And (Not scenarioType.XFEqualsIgnoreCase("ScenarioType1")) Then
If entityText8 = "" And (Not api.Entity.HasChildren()) Then
Return ConditionalInputResultType.NoInput
End If
End If