Can spices, like cloves, be reused?

Much of the flavor and aroma of spices come from volatile compounds. Once these compounds are released by heat, grinding, interactions with fats, etc....they are mostly given up to the product they are added to, and to the environment. Even without these physical and chemical disruptions, these volatiles dissipate over time. That is why it is recommended that cooks purchase whole spices, in small amounts. Having said that, there are some potential applications pointed out in other answers. One just has to expect that the impact of the re-used spice will be different...in some cases, quite dramatically, because of the loss of volatiles.

One thing that's quite standard is to use vanilla to make vanilla sugar, then extract (with alcohol). The pods can finally be cut up and added to custard or ice cream.

This should work with many sweet spices, such as cinnamon sticks and cloves, that can be put whole in sugar to give up some flavour. The flavour of course reduces at each use, and not all will give up flavour to sugar very well. Star anise might be worth a try, as might cardamom (seeds not whole pods, remove by sieving) and juniper berries. Nutmeg probably not. Mace, black pepper, allspice - maybe, but I suspect the flavour won't be worth it.

Some of the same spices could be used, dried, and used once more in something similar, but both the first use and the drying cost you flavour, so you probably only get 2 uses before it's not worth it. Spices for mulled wine might make a few batches over the course of a day, but you'll need to top them up - or think in terms of one continuous batch.

Think of spices like coffee. There's only so much flavor in a coffee ground, whole or ground clove that can be extracted, once it's gone that's it. Secondly, once you expose a dry spice to water it rehydrates, and spoiling sets in.

The only way I could see reusing whole spices would be if you used them briefly, then reused them very soon afterwards. Otherwise you're not getting the most out of them and then they'll go bad on you. I can't think of many cases where that would work.

Hot peppers can be reused for a long time. I just dip one in any sauce that will go into my food, wash it off if necessary, then put it back in the fridge.

It dries up gradually, but stays spicy. Anything over half a mil scoville should serve a regular person for a couple weeks.

Yes—whole spices like cloves can be reused once (sometimes twice) if they weren’t cooked to death. Expect weaker flavor; steep longer or add a few fresh ones. Don’t reuse ground spices. If the spices sat in perishable food or smell musty, discard.