When you delete or reject a return, your customer will not be able to request the same return a second time. This also applies to returns that you archive.
We're currently looking to modify this to allow you to "cancel" a return, which would allow your customer to request a return for the same product a second time.
Archived returns will never be reset however.
Note that deleting, archiving, or rejecting a return will not erase it completely from the dashboard. You'll be able to search for the return number in your search tab. A record will always remain :)