Here's a partial list of things that will cause the elves to dislike you. Note that if they start bringing non-cloth items, they might be getting tired of your nonsense:
1) Collect some firewood. The more logs you have in your stockpile, even if the elves themselves gave the logs to you, the more grumpy they'll be when their diplomat comes by. Note that you need at least a baron.
2) Trade them stuff. Things coated in non-goblin/kobold blood ticks them off if I recall, and wooden things (and wood derivatives, such as green glass) will similarly infuriate them. It's possible this isn't actually effective... unless combined with
3) Loot. Loot, loot, loot. Take everything on the caravan without giving them an Armok-damned dime. This one will actually work with the humans, too, eventually.
4) Walls... Once you take everything that could theoretically be of value, wall up the trade depot and receive quality entertainment as the elves and their mounts gradually go insane and murder one another. Then sell their bones-made-crafts to the dwarven caravan. Again, this works with the humans.
Ticking off the humans isn't as good as elves, since they'll actually bring real weapons instead of toy (don't allow near small dwarves) wooden swords. And you get to actually select purchases from them, making them vaguely useful. Especially if you're trying to buy more wood to annoy the elves.