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Easy to do, as you see someone eating or drinking begin to cast arcane missiles at them but as soon as you begin to cast, stop casting. This will make them stand up and force them to stop drinking or eating.
Fun thing about this is if people are drinking or eating near guards (for example at the isle of quel'danas), you will not get aggro from them as you never enter combat and neither do they, so you're pretty much making them waste money, tested and working on live, enjoy
*edit*
Changed from useless because it's fine.