Your threats to devastate the city make me think that in Rome the police was right. I repeat, in Rome nothing have ever happened with other supporters, except with you. So, maybe the problem is within your typhus, not in the roman police.
The authorities want to prevent Feyenoord supporters to go to Napoli (yes, it's official) for safety reasons, beacuse of what they did in Rome. So you have to blame only your friends.
In any case, in a city like Napoli where the local authorities have to deal every day with the Mafia, do you really think that they get scared by some dutch vandals?
Napoli is not Rome, if they will go there despite the ban, I can assure you they will have a really rough time.