Click the 'Filter by object type' button (next to the search box, in the 'Objects in Project' pane). Choose the objects that you want to hide in the diagram.
Right click on one of the objects and choose Select > Same Type. This should just highlight the objects of the same type e.g. all of the stored procedures.
Right click on one of the objects and choose Hide.
This should hide all of the objects of that type from the diagram.