We had a similar requirement. Our solution was to add a new flag InfoObject to the underlying cube.
There was a routine implemented in the transformation, which would set the flag to 'X' if the record should be excluded (i.e. if it belongs to the unwanted plant/material type combination). If the record should not be excluded, the routine would set the flag to blank.
Then in the report, we added a filter on the flag InfoObject, so the report would only show records where flag = # (blank)
The advantage of this design is that in future if somebody wants to see all the data without any exclusions, he just has to remove this filter.