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I think that I have done so the callback is being removed, it works when I don't leave the map, but when I reenter the map it starts again.

AddEntityCollideCallback("Player", "ScriptArea_2", "CollideShock", true, 1);

Sure does "true" mean that the callback should be removed after being used once?

Where is that callback being set in, OnEnter or OnStart? You want it to be in OnStart. OnEnter gets called every time you enter the map. OnStart gets called when the map gets loaded within the session for the first time. You also want to think about any other callbacks you have set in OnEnter.