Here is what you do ( the long and effective method ) , hoard all of your tree/animal byproducts into an area of your fort that doesn't get any elf traffic, and buy them out of all of their possessions, and then start to give them tributes.
If you please the elves they'll come back next year as a larger caravan, make sure to stock up on supplies, and troops for D-Day.
After a few years of pleasing them, you should place the depot in a place that is filled with animals remains, and logs.
Be sure to equip a militia with hammers, and place foods and drinks around the depot, make a lock system to make the elves enrage.
It can take you many years to make this set up, so make sure that this depot is inaccessible until it is finished.
When the D-Day comes, lock up the large caravan of elves, and seize all of their goods, after you injure most of the elves set them free, so that they can give their pansy friends a little incentive for them to attack you.
Afterwards you need to continue to displease the elves some more, just that they can get the vibe.
Please elves > They come back as a larger caravan > Stock up on supplies > Prepare nightmare depot > ... Repeat... > Supplies are sufficient > Evil Depot Complete > Open Evil Depot > Large Elven Caravan comes into evil depot > :C > Seize all goods > Lock them up > Enragement > Free the cripples > Elf Civ get's message from cripples > [Insert Displeasement Tactcs] > Repeat > WAR!!!