The two sides met in the European competition in May with West Ham emerging victorious in their semi-final clash.
But in the second-leg at the AFAS Stadion in the Netherlands, AZ fans tried to break into the away stand where West Ham supporters were.
It was also where the West Ham players' friends and family were sat.
Members of the Hammers squad even tried to intervene and bat away the onrushing hooligans.
It led to chaotic scenes in the stadium, where one viral video saw West Ham fan Knollsy become an overnight legend for holding a group off from entering the stand.
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Despite the serious nature of the attacks, AZ Alkmaar have escaped the wrath of UEFA and been given a paltry fine for the madness.
They will have to pay just £68,670 (€80,000) in fines, which has been broken down into a series of punishments.
The fines are as follows: £6,115 for throwing objects, £12,870 for lighting fireworks, £6,867 for blocking public passageways and £42,918 for crowd disturbances and failure to ensure that spectators cannot move from one sector to another.
AZ have also been ordered to play one European game behind closed doors - though this has been given a suspended for two years.
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Despite the horrific scenes in the Netherlands on that night in May, the Eredivisie outfit will be allowed to play in next year's Europa Conference League - which they have qualified for.
Had West Ham fans and players not intervened, it is not known what would have happened had thugs reached the various families in the stand.
Michail Antonio, Said Benrahma and Lucas Paqueta were among those to jump into the stand to defend their families.
AZ have handed out stadium bans to 43 fans while a Dutch television show made pleas to the public to identify those responsible.
West Ham had won the match 1-0 after Pablo Fornals netted late into the contest to finish the scores 3-1 on aggregate.
The east London side went on to play Fiorentina in the final, who they beat 2-1 with a last-minute goal from Jarrod Bowen.