Re: trading with elves - the moment the elves get annoyed and 'stop trading', you're still in the trading screen and can still 's'eize all the goods you want from there. (And you can still rob them by demolishing the depot or murder them.)
My own fort:
Once again, i gave too many old clothes to the humans. Except this time, it must've been perhaps four or five too many: when the last wagon of the human caravan was four steps from the exit, the dwarf caravan showed up and piled right into it. Gah! Built a secondary depot, cleared a way to it, and once it was sufficiently advanced in building, i forbade the main depot. Got the dwarf caravan unstuck from the unfortunate human wagon and allowed it to leave. Unfortunately, i couldn't get the dwarf caravan to congregate at a single depot, so had to trade in separate sessions on both depots, and _of course_ the goddamn loonies wouldn't clear out the shit they had unloaded (and acquired) in the main depot. Half a dwarf caravan seems to carry something close to a hundred thousand in value.
I did with the unwanted gains what i always do in such cases: build a bridge, designate a garbage dump under it, order all 'stolen' goods dumped there, pull the lever. It took half a year to shift all the junk, but at least the result of the lever pull was very satisfying: my 'imported goods' rating went from 120k to 41k in an instant. Oh, and the emergency depot was demolished as soon as the dwarf caravan was gone.