How to hide whispers indefinitely?

Hi, Whenever any player looks at "View all chat entries for this game", all whispers appear, as well as all gmrolls. If I choose "Hide whispers", it only applies to me, not to the players. This is really bad, since whispers and gmrolls are private only during the session in which they are said/rolled, and after that they become public: Huge spoiler. Any ideas on how to fix it? Thanks in advance for your help.

Save your chat history locally and then delete your chat history after each session?

Yes, thanks. Not the best one, but a solution after all. However, I don't even happen to find a way of deleting or hiding the chat history. Could you help me with that? 

go to your campaign details page, click settings, select "delete chat archive" a dialog box will appear, and you must type "confirm"

All right. Thanks!

Question to the original poster. Are saying that everyone in the group can see all the whispers? The reason I'm asking is that whispers are only seen by the GM (including co-GMs) and the sender or participants of the whisper (that is for group whispers). To a GM it might look like everyone can see the whispers but it is not suppose to be. If a player has stated they can see whispers that they were not part of the discussion, please post a bug report in the  bug and technical forum so that the appropriate people can look into this.

It seems to be the case: In "View all chat entries for this game" all whispers are public to everybody. I will check again with actual players before reporting a bug, but one player came to know about stuff he shouldn't, and it could only have been through reading (old) third parties' whispers. If I re-enter as a player, I am able to read everything as well.