Is there anything a site can do to protect itself from incoming and outgoing
datagrams with spoofed source addresses. Explain

On the outgoing datagrams from your company, you can limit it on your
switches/firewalls depending of your infraestructure.

In general currently there are not global solutions, please read a
brief article on this subject:
http://www.securityfocus.com/infocus/1674

There isn't much you can do to detect or block them. You can (and
should) reject outbound packets (from your network to the Internet)
which don't have a source address of one of your networks. You should
likewise reject inbound packets which have a source address of one of
your networks. Other than that, there's not much you can do. If you
are concerned about packets originating and ending up within your
network, you can implement similar filtering on internal routers,
switches, or firewalls (ie. only allow a site to send packets whose
source address is from that site.)