What can you do with leftover shmita wine, for instance if it was spoiled or contaminated by a foreign substance?

Shalom, Thank you for your question. You are correct in being concerned with the leftover wine that has kedushat shmitah. This wine (which I assume was purchased via an Otzar Bet Din system, where wine with kedushat shmitah is distributed to the public) must be treated with “holiness” and can not be thrown away. However, once the wine is no longer fit for drinking, it looses it's holiness and may be disposed of. If it is already not drinkable you may pour it down the drain. If it is still drinkable (but perhaps you do not desire to drink it, because an insect fell into to etc), you may leave it out with the bottle open for a period of time until the wine spoils, and then you may dispose of it. Blessings.