Napoli have been fined 150,000 euros (£120,000) by UEFA for poor stadium organisation, ignoring security directives and the inappropriate behaviour of their supporters during a Europa League match.
The European governing body has also threatened the Serie A club with playing a European match behind closed doors if the issues at their Stadio San Paolo resurface within the next five years.
A UEFA statement said: "Italian club SSC Napoli have been punished by the UEFA Control and Disciplinary Body for the inappropriate conduct of their supporters, insufficient organisation in the stadium and non-respect of UEFA directives related to stadium security at their UEFA Europa League Group F match against Swedish opponents AIK on 20 September."
Napoli will also be banned from selling tickets for parts of the main stand for their next European game at home to Dnipro on November 8 unless they can provide "a report from an independent construction company which confirms, with photographic evidence, that the necessary repair work has been carried out and that the aforementioned stands meet all the relevant spectator safety standards".
UEFA said if satisfactory repairs have not been carried out by November 6 they could prevent European matches taking place at the stadium.
Napoli have three days to appeal the decision.