Remove a custom group/button from the ribbon

[Andy Pope]

I don't have Project in order to test but I think the issue is a missing space in the string you generate.
You need to seperate customUIxml from the start of the schema namespace,
mso:customUIxml ns:mso

Sub RemoveCustomUI()     
    Dim customUiXml As String      
    customUiXml = "<mso:customUIxml ns:mso=""http://schemas.microsoft.com/office/2009/07/customui"">" _        
    & "<mso:ribbon></mso:ribbon></mso:customUI>"      
    ActiveProject.SetCustomUI (customUiXml) 
End Sub