A facility called Ping is quite legally obtainable for all users to sort out your own computer malfunctions. What steps are taken to stop other users targetting your computer to send a stream of pings so making your computer almost unworkable?
Bandwidth.. The originator would need to have a bandwidth far far in excess of the computer they were trying to flood otherwise it just wouldn't be noticed...
Also most gateways are set not to respond to pings on the external interface.