AZ Alkmaar have been fined 80,000 euros (£68,670) for the behaviour of their fans at their Europa Conference League semi-final second leg match against West Ham in May.
The Hammers, who won 3-1 on aggregate, went on to win the tournament.
During that game at the AFAS Stadion in the Netherlands, AZ fans attacked an area in which West Ham players' friends and family were seated.
Players tried to intervene to protect their family members.
Michail Antonio and Said Benrahma were among those who tried to step in, moments after the Premier League team had reached the final.