You can't avoid DoS attacks completely just so you're aware.
All you can do is make them less effective, this company unfortunately had absolutely zero protection so it received absolutely full destruction of a DoS attack.
The protected ones will still slow the servers (How slow depends on the power of the server itself), but when Sam finds another host, if it's powerful enough the DoS attacks will make such a small impact that they won't bother anymore.
Pick a too weak of a server, and they will continue to attack because its working....attackers don't attack if it doesn't work