Prevent spoofing of assigned IP addresses

[Tinfo]

A range of IP addresses are assigned to a company, but only some of these assigned IP addresses are routed onto the Internet. Clever spammers seem to be able to find somehow the IP addresses, which are not routed onto the Internet. They then spoof these IP addresses and use them then to send out spam e-mails. Now the company to which these addresses are assigned, says that the spam e-mails did not originate in their organization, because these IP addresses are not routed onto the Internet. This company, however, also says that they cannot do anything to prevent the spammers from finding and from spoofing these IP addresses. Is this so? What can and should this company do? Thanks!