It's caused by the trait called Hero of StrangerVille, which comes when you complete the StrangerVille Mystery aspiration. As it is a reward trait, re-traiting potion won't help. So you must use cheats.
First you need to enable testingcheats:
testingcheats on
And then you have to enter this cheat:
traits.remove_trait trait_HeroOfStrangerville
When the trait is gone, your sim should no longer receive fan mail.
Comments
In the rewards list there’s a re-traiting potion. There might be cheats you can use to remove it if you don’t have sufficient points for the potion.
First you need to enable testingcheats:
And then you have to enter this cheat:
When the trait is gone, your sim should no longer receive fan mail.
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