Wales keeper Lewys Benjamin made a crucial save to deny Stassin a second goal three minutes into the second half.
But from the resulting corner Romeo Vermant powered home a header to extend Belgium’s lead.
Vermant came close to scoring his second soon after, curling an effort past the post while Cameron Congreve fired over the bar at the other end.
Wales were reduced to 10 men when Ed James was shown a second yellow card for a challenge on Vermant.
Stassin scored his second and Belgium’s fourth when Mbangula’s corner fell kindly for the Saint-Étienne.to head from close range.
The impressive Moreira scored Belgium’s fifth with a strike from outside the area which gave Benjamin no chance.
Sam Parker conceded a penalty for a foul on substitute Stanis Idumbo and Norman Bassette converted the kick.
Wales’ misery was compounded in stoppage time with Noah Adedeji-Sternberg’s netting a seventh goal for the visitors.
A second successive heavy defeat leaves Wales at the foot of Group I ahead of their next game, away to Austria on Tuesday, 14 October.
Only the group winners qualify automatically for the finals with the runners-up going into the play-offs.