Make it so that the curse is still tempting. He'll discover the curse, but make the item useful enough that the party will keep it around just in case the curse is worth it.
Requirements or Drawbacks are good ways of doing this. A Sun Blade that causes a constant solar eclipse in a mile radius, a healing wand that moans in pain and requires you wound yourself for 5HP each time you use it, or a staff with a statuette on top that weeps blood and must be washed in holy water to make it work.
You'll want to balance the item's utility and curse so that even knowing the curse the person is tempted to use it, but not make the item so useful that it's automatically worth dealing with the curse. I find that this temptation is stronger if you give your players exact numbers: "Each time you use it roll a d20 and the curse triggers on a 1" is far more tempting than "there's some mysterious chance that the curse triggers whenever you use it."
Items like this will be far more effective if you make it clear that there's no curse compelling them to keep using the item. Something is much more tempting if you can quit at any time. If they need to use remove curse to get rid of the item, most players will immediately reject it.
A kitsune using the feat Fox Shape does not retain the use of his pair of demon talons...
The section Magic on Transmutation on the subschool Polymorph says
When you cast a polymorph spell that changes you into a creature of the animal, dragon, elemental, magical beast, plant, or vermin type, all of your gear melds into your body. Items that provide constant bonuses and do not need to be activated continue to function while melded in this way (with the exception of armor and shield bonuses, which cease to function). Items that require activation cannot be used while you maintain that form.
For the item (even an attached item and likely any attended item) to have an effect on the assumed form, the item must both provide a non-armor or non-shield bonus and not require activation. (The use of the word that in the emphasized sentence is lone a complementizer making the pair of phrases that follow simultaneously (not independently) modify the subject items. The sentence needs a second that between and and do for a different reading. And I totally agree that rules should be clear without having to go deep on grammar to be understood.)
Thus, while the demon talons don't need to be activated, they also don't grant a bonus, so they are nonfunctional when the fox shape is assumed (and were the talons magicked further to grant enhancement bonuses on attack and damage rolls, that wouldn't help either).
If it's any consolation, unless the kitsune picked a fox shape that was missing both forepaws when he picked the feat Fox Shape (which, I guess, is possible but really weird), the feat's transformation into a fox gives the kitsune all its paws despite his normal form having had the hands tragically (or voluntarily) removed: the fox shape's "appearance is static and cannot be changed each time you assume this form."
...But a house rule allowing retention probably won't unbalance the game
A kitsune who spends 40,000 gp on a pair of demon talons should see if he can convince the GM that the talons, being attached, are no longer gear but just limbs like his hands were. Allowing the kitsune's fox shape to keep the natural attacks granted by the demon talons, on its surface, doesn't strike this GM as particularly unbalancing.
Best Answer
If some specific cursed items can be destroyed (rather than taking the suggested steps for their removal) depends on the campaign
While randomly cursed items are quirky, randomly cursed magic items' powers are otherwise determined normally, they are otherwise employed normally, and they can otherwise be destroyed just like the normal versions of those magic items. A pearl of power that can only be used by a creature who worships a specific deity remains, at its core, just a pearl of power. (Randomly cursed items are described on pages 272-3 of the Dungeon Master's Guide.)
However, specific cursed items require DM adjudication on an individual basis. For example, the boots of dancing (DMG 272) (30,000 gp; 0 lbs.) says, "Only a remove curse spell enables the wearer to be rid of the boots once their true nature is revealed." What this phrase means exactly is up to the DM, and the DM has several options.
…And countless variations of the above and entirely new options besides.
In the end, it's up to the DM. There's no way of knowing if a specific cursed magic item can just be broken or lit on fire so that its curse is lifted, bypassing the need to get a remove curse or similar effect, without also knowing how the DM intends to handle cursed items in the campaign.