"Sherlock, you're going to do what?" John asks.
"Write a letter to them," Sherlock replies, pointing aimlessly at the laptop on the desk, obviously anxious at the prospect of people reading this.
"Who?"
"The people in there," he says, pointing again.
"What people?"
"The people who are reading this right now."
"Reading what?"
"Reading that!" Sherlock once again points at the computer, this time more distinctly so that John would actually look at the screen.
"Is that what…" John starts. "Hold on a minute." He stares more closely as words appear on the screen as if someone is typing them. John looks confused and then suddenly alarmed. "Hold on a bloody minute!" These words appear on the screen as soon as he says them. In fact, this whole story is an account of what is happening in their flat at this very instant, and this is what they are reading on the screen of the laptop. "So what you were saying earlier is true, then?" John asks, still utterly bewildered.
"Yes, whatever we do ends up as a story posted on the internet, and as soon as I discovered what was happening, I decided to write a letter to them explaining…things." Sherlock struggles to find the right word. He himself is, in fact, unsure of what he would write. "That's why I asked for your help. As you have pointed out numerous times, I'm rubbish at the whole 'polite' thing."
There is an awkward pause floating about the room, but it is brushed away after John finally takes in what Sherlock has said.
"Well, let's start on the letter, then," John says to Sherlock.
He starts typing.

Dear Readers,
I want to let you know that I know all about your stories about the lives of myself and John Watson. You know everything about me, and yet you've never met me. This irks and deeply disturbs both of us.

"Come on, now! Don't drag me into this!" John says. "You're also not polite enough. Keep in mind who we're talking to. I don't want to give them a reason to hate us."
Sherlock visibly ignores him and continues typing.

I politely invite you to please stop doing this, pretty pretty please, from the bottom of my heart, please stop.

"That's… No, Sherlock," John says. "I think you should make it sound like you. A polite version of you, of course, but still you."

If you truly are a 'fan' of my work, then surely you will understand my plight. I do not like being the subject of these 'fan' fictions, 'fan' artworks, 'fan' videos, and other such 'fan' creations. Stop, and if you do not stop, then I will personally hire someone to find you and force you to stop. I am Sherlock Holmes, after all, and I know several criminals who would be more than willing to oblige.

"Now that's just… too much," John remarks. "Maybe you should try an average approach, something a normal person would say."

Your horrendous accounts of our lives, some true, some partially true, and some incredibly distanced from the truth, have angered us both, and we politely ask that you refrain from any such activity in the future.

"Wait," John says cautiously. "What do you mean when you say 'incredibly distanced from the truth'?"
"I mean," replies Sherlock, "that they make up things that are not remotely true and post them alongside the ones that are true. I have seen some of these false stories, and they truly are horrendous in the worst, most inappropriate way possible."

I do hope that you reconsider your choice to post such things online in the future as there is no telling how we may react.
Sincerely,
Sherlock Holmes

"So," John says as Sherlock finishes the letter, "do you think that this will make them stop posting about us?"
Sherlock smirks and, without taking his eyes off the screen, replies. "Not at all."