When I hide columns in SSRS they still appear in the CSV export.
I have to hide columns, not the entire tablix.
This is what I have tried already: The filters in the tablix hide rows not columns. The DataElementOutput per column can not be set using an expression.
Format options such as expressions on visibility are ignored for CSV rendering methods. CSV rendering methods are essentially data flows, so you can suppress elements that you don't want to include in CSV files by changing the DataElementOutput from Auto, the default value, to NoOutput.
There is a solution in the version of SRS 2008 R2, you need to perform a hide fields as follows:
=IIF(Globals!RenderFormat.Name="CSV", True, False)
means this part of the "Globals!RenderFormat.Name" is crucial
来源：https://stackoverflow.com/questions/6511512/ssrs-2008-hidden-columns-should-not-export-to-csv