Click on your Matrix, and you'll see a Grey border area around it. In the top left there'll be a small box. Right click that and click "Tablix Properties". Then go to the Filters Pane, and you can add filters only for that Matrix (which, if you wish, can be based off parameters).
Thom~
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.
Larnu.uk