The informant can still communicate with you, and you with them, even if the report has been closed. The communication is only blocked once the report is archived, and a retention policy has been applied. Once the case is archived, the informant can see a message explaining that they can no longer communicate following the archiving. They can still view their report and messages, and download the report in PDF.
Once the report is deleted (following the expiration of the retention period), it disappears from the inbox entirely.