How to prevent SharePoint users from discarding check out on behalf of other users?

This issue can be resolved by managing the SharePoint permissions properly.

Any user who has following level of permission on the document would be able to discard the checkout done by other users:

• Full Control 
• Design 
• Edit 

So, to stop users from discarding check out, you should add users in site-members group instead of owners group.

Appropriate permission level: Contribute.

If required then a custom permission level can also be created to get the desired result.