If you don’t want other users to share your posts on their stories, you can disable this function for them.
Here is a step by step guide to stop people from sharing your posts on their stories.
1- Tap the head and shoulders icon (profile icon) on the bottom right to view your Instagram profile.
2- Tap the three dots icon on the top right of the screen (Android) or the gearwheel icon next to “edit profile” button (iOS).
3- Scroll down the page to find “Resharing to stories.”
4- Tap the “Resharing to stories” and slide the switch to the left to turn it green (off).