While gluing down some static grass onto my diorama, I realised that the PVA, although clear, leaves a slight shine once dried. How do you get rid of this shine easily?

The easiest way would be to spray the whole diorama down with some flat coat of some sort. Either with airbrush or with a spray can. If this is not something that's not available to you for some reason you could try giving the whole area a wash with a flat ground colour. Flat brown or green or black or a mix would do it.
If you haven't done a wash before the typical way would be to dip your brush in paint and mix it with lots of thinner, such as white spirit. Liberal amounts of the thin mix should be applied all over the model.

K

Those previously stated responses would be the best ways.

You can tint the PVA first next time,but will still have a glossy sheen to it. Next time you can sprinkle on sand to give it texture. This will make the sheen a bit more unnoticeable.