Artem,
Personally, I would recommend applying certain statuses to your False Positive incidents and running a weekly query on those incident and then just manually deleting the messages.
You may be able to write a script that looks at your Oracle database and removes all messages based on status or something but I don't know how you would specify whether or not the entire message gets deleted or just portions. Always use caution when looking at database options because it's never a supported method from Symantecs eyes when throwing custom scripts at the database so I would'nt even attempt it.
Try submitting an enhancement request under the Ideas section here to get that introduced because I think it's a great idea that could save on storage in the future.