The Italy international launched an expletive-ridden tirade against his former club Leicester Tigers following Toulon's 25-21 defeat in the European Rugby Champions Cup earlier this month.
Castrogiovanni tore into Tigers director of rugby Richard Cockerill and branded his previous employers "amateur" after storming into a press conference.
The 33-year-old, who made 149 appearances during seven seasons with the Tigers, was on Wednesday found guilty of misconduct by European Professional Club Rugby (EPCR).
Castrogiovanni was punished with a four-game ban, suspended until April 2016, and his fine will be donated to a Leicester-based charity.
An EPCR statement said: "After considering evidence and hearing submissions on behalf of Castrogiovanni, who pleaded guilty, and on behalf of the EPCR Disciplinary Officer, the independent Disciplinary Committee found that Castrogiovanni was guilty of misconduct and that the comments made were utterly indefensible.
"The committee chairman, Antony Davies, accepted that Castrogiovanni's comments regarding Leicester Tigers being "amateur" were made in an ironic and questioning manner, and therefore did not constitute criticism. He also acknowledged the player's genuine remorse.
"Castrogiovanni was ordered to pay €10,000 to a charity in the city of Leicester and was banned for four matches with the ban suspended until 30 April 2016.
"The ban will be activated in the event of any off-field misconduct complaint being successfully brought against him. He was also ordered to pay costs."