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So, with a script, you set it so that it can only be visited by a specific player, but it turns out that it's visited even later by hired heroes?
MarkObjectAsVisited (object,geroi);
By the way, there was another option: leave the dolmen enabled, add a trigger, and use a mod to remove the +1000 experience. Do you happen to know where this file is located and which line to change in it? (e.g., to make it give a minimum of +15 experience)?
That would be an even better option :)