It's often as simple as "I've read the internet already, please stop forwarding me these old jokes". Invariably followed by them with "but I saw this one the other day about $blah and.." where you cut in, summarise very briefly, and skip to the punchline. A few iterations of that normally gets the point across. A few iterations of them finding people like this, and they realise there's not a whole lot to be gained by forwarding them, especially if they've got 20 levels of indents already.
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the hatterthe hatter