Can I remove unwanted symbols from my Main Model diagram?

I've imported a very large data model  (1082 entities, 2395 relationships) from PowerDesigner, and the Main Model diagram is pointless. The submodel diagrams are the ones I expect people to focus their attention on.

The model takes longer than I would like to open and close, and I have to be careful when I select an entity in the browser (it automatically shows me where it is in the diagram, - it takes 2-3 seconds each time, which is annoying), so I thought maybe I could improve performance by deleting all the symbols on the main diagram. I'm only able to delete the entity, not the symbols.

Is there something I've missed?