This is Il Sonno degli Dei by Jack McDevitt (original title, The Engines of God). In Italy it was published by Urania, and this is the cover:
(you can still find several copies on eBay).
- they are not spheres, they are spherical clouds (dubbed "Omega clouds")
- they come every 8000 years, roughly. They "come from the Void" (the inter-arm empty space towards the Galaxy center). Last time on Earth was around 2500 BCE (the archaeological remains of Mohenjo-Daro), so next visit is around year 5500 CE. If memory serves, we are now around year 4000 CE. Moving at around 5% of light speed, the nearest Omega Clouds are about 75 light-years distant, hopping from habitable star to habitable star.
- they attack whatever presents sharp corners, which is a telltale of technology
- the story ends just like you remember - they are orbiting around another planet, Quraquat; in the Italian translation, the guy (I think the copilot) says something to the effect of, "we still have a good one thousand years, that's time enough", and the protagonist looks at the planets, shudders and says, "forse è quello che pensavano anche loro".
The scene you remember takes place on a frozen planet, in a system where they have located an incoming cloud and, to test the hypothesis, land and use a big laser to sculpt an ice mesa into geometrical shapes. Only, they're flying in a geometrical-shaped shuttle which too gets attacked.