I've seen it happen on occasion even on public networks. The worst outcome I've seen is that the ISP of the offending network blacklists them.
Internet addressing is not really a legal consruct per se unless and until the behavior could be demonstrated to be willful and damaging. In such a case the usual laws that exist to protect business and individuals come into play.
I don't have a citation for you and I am by no means qualified to give legal advice so please don't interpret this as such. It's just my layman's understanding based on over 30 years of being a network engineer.