I'm a bit confused about how destroying it would be beneficial.
The idea behind boycotting things is to make them less profitable to produce.
You don't buy a stolen computer because you are giving money to a theif. That gives the thief incentive to go steal more. The fact that the theft already happened does not make it ok to purchase it. But only because it gives incentive to the theif.
If the thief already sold it to someone, he has already gained the maximum profit from it he will ever get. And if the computer gets destroyed after he's already sold it, well all the more better, that's one less used computer flooding the market and possibly one more dude looking to buy a computer.
If you have a receipt, you could return it, that would harm the vendor selling ivory. But just destroying something after it's already been bought and paid for isn't going to help or harm the poaching industry at all.