I keep running into an issue as a dungeon master where my players develop their characters a bit too much. By this, I mean that they'll write up things in their backstories or develop NPCs that aren't their own, and decide things that their character does not know.
Specific examples:
Player 1 has a character who was saved from drowning by an unknown sea entity which left him with a cursed sword. Great! But now player 1 will occasionally try to send me ideas of what he thinks may have been the monster-- which is really not up to him.
Player 2 is playing a character that she played in a previous campaign, which I'm allowing because she was more or less expelled from the campaign by the DM refusing to do his job. It's fine, but her character has a developed parent figure that she's written out text RPs with (which I don't allow in my games, but others have.)
We've played 7 sessions. I'm running two groups with the same campaign, but both are at 7 sessions at the moment.
I don't think either of them is doing this maliciously or to try and subvert me as a DM-- they're probably trying to help! I don't want to be too rude to them, but I do need to take the reignsreins back.
How do I discourage this?
How do I take control of unknown backstory entities with players who keep overthinking things that they have no control over?