Yeah but f.. off simply does not say the same thing that his letter says, now matter how succinct.

He writes like he assumes good faith, then explains why he thinks that exactly this attempt won't be fruitful, giving a good-faith argument for why Oswald should consider further correspondence fruitless, unless he changes his whole political ideology.

That's a lot more than just "I don't want to talk to you and I think badly of you"