Fuimaono-Sapolu was last month found guilty of accusing referee Nigel Owens of bias in comments posted on Twitter after Samoa's World Cup defeat by South Africa.
The six-month ban was suspended for two years on three conditions - and came with a warning that it would be triggered if he is found guilty of misconduct within that time.
An appeals committee upheld both the verdict and the suspension on Saturday.
The IRB will now consider whether the three-week ban Fuimaono-Sapolu received for comments aimed at Saracens fly-half Owen Farrell on Twitter will trigger the full six-month suspension.
The only concessions the appeals panel made to Fuimaono-Sapolu were to extend the timeframe for him to complete a refereeing course and to adjusted his requirement to complete 100 hours community rugby work in Samoa, taking into account his existing projects.
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