can I report people for this action?

No, you can't. There's nothing against buying certain items on certain champions. Doesn't matter if it's ranked or normals or even aram. All you can do is ask why they're building it and maybe suggest something else depending on the response. But that doesn't mean you always should.

Some item builds work, others don't. All you can do is see how well the person does with the items and if it's not working well then you SHOULD suggest they try a different item(s). But expect a toxic response if you do, some players don't take criticism very well.