So...whatever I vaguely recall as the truth was false? Then why does everyone kill elves?
In previous versions, people killed elves because it made them bring more animals. This is believed to be because the elves used only pack animals (which had a much more limited carrying capacity than now), and so had a very limited carrying capacity. As such, if they liked you, they'd try to bring lots and lots of stuff, but not be able to fit it on the pack animals, so they loaded up with binned items (cloth) first. If one were to piss them off by murdering a bunch of them, they'd stop wanting to bring so much to your fort, so wouldn't be able to totally fill their animals' capacity with cloth, so would bring more other stuff, like exotic animals.
Nowadays, since pack animals' carrying capacity was so greatly increased and most items' weight decreased, it's virtually impossible for them to want to bring so much cloth that nothing else gets loaded. As such, now people kill elves for being cannibalistic hippie dickheads. Which, to be honest, was the main reason people killed them before, but oh well.
As Quietust mentions, it also appears that the amount of cloth they bring is intended to be "emergency supplies," similar to how they bring less wood if you've got lots. So, leave a bunch of cloth unused and unforbidden in your fort, and they should bring less of that. It's unlikely to make them bring more useful stuff, though, since the useful stuff the would've brought probably actually was due to the aforementioned carrying capacity increase.