When it comes to serving jam to your guests, honesty is the best policy. Literally. Just say it like it is: "Hey, this jam is going to give you a stomachache, but you're still welcome to have some."
Read more about the Overly-Honest WarningBy placing a large trampoline in the middle of the room, you can encourage your guests to burn off the calories from the jam before they even get the chance to eat it.
Read more about the Jam-Filled Trampoline of DeathBy creating a series of memes about the dangers of jam, you can subtly (or not-so-subtly) warn your guests about the impending doom that is the jam.
Read more about Jam-Proofing MemesBy setting an alarm clock to go off every time someone approaches the jam, you can create a sense of urgency and panic, making it more likely that your guests will avoid the jam at all costs.
Read more about the Jam-Smelling Alarm