Look nice and still tell people to sod off

• take someone's telephone number and suggest calling them back later
• bring them something relatively meaningless (cup of coffee usually suffices) and then cut the meeting short
  
• get called away (my phone has an alarm/reminder function which I can set to any time I like; stopwatch also works)
• any of the following phrases work: "I'll look into it"; "I need some time to discuss this with the team"; "the person responsible is not in at the moment, do you want to leave a message?"

I think I prefer this subtle route - which makes people feel empowered, while still getting shafted - rather than the blunt route, which is more honest and truthful, but also makes people angry directly.