the stun ability (Shock Powder) hits other players even when PVP is disabled

What you mention can happen when players are put into different teams.

This is not possible for players without teams.

The default configuration is PVE oriented, friendly towards villagers and other players.
Put players into different teams to allow hurting each-other.
Alternatively you can change this in Spell Engine server config (config/spell-engine/server.json5)