No, that's not what I am saying because this would fall under "cheating - unsporstmanlike behaviour" - see Rules of Use and it should be reported. 
Kind Regards :)
John

Hello stephane", I am absolutely sure! 
When you use the IN GAME mute function - as described above, All spectators are muted for Both players. 

I tested this this and and I used it several times when playing memory.
In some cases my opponent complained because he wanted to speak to the spectators but could not do this anymore because i used that IN GAME mute function. 
Have a nice week-end!
Kind regard,
John

-to the 2nd point
- you are saying it is not against the rules if a chess grand master decides to watch a game and tell one of the players( lower ranking) which moves to make?

No, I don't know that.
I know it's available for: Chess, Reversi, 8-Ball, Backgammon, Memory, Checkers. 

Unfortunately I don't have time to check this for all the other games right now. 

In case you are going to check this, please let us know. 

Thank you and kind regards,
John

You are very welcome and I concur. 
I gave him advice on how to stop the interference and how to report unsportsmanlike behavior. (Since he doesn't have a dice yet and not the three necessary dots on it, reporting is currently only possible with the provided link.)
Take care Stephane"
John

Is there a way to speak privately? Having someone that stalks me read our conversation in the lobby isn't a great option.

Hi 4thDan,
To silence spectators, click on the gear icon and select the 'mute spectators' option.
see screenshot:  --> 

https://mega.nz/file/61MSAJyB#Ms0P0PTXtNjrebN1QcEHaX0b5zGsKc3UwoxbWPUlfE0

The spectators' writing is invisible to both you and your opponent if you've done this.

Watching games or conversing with players is permitted - please refer to FlyOrDie's Rules of Use, which you agreed to when you started playing here.  --> Rules of Use 

https://www.flyordie.com/games/help/moderation.html

If you need to report "unsportsmanlike behaviour you can use this link: 

https://games.flyordie.com/s/reportAbuse.html?portal=flyordie_com&lang=en

Kind regards,
John

Hi John , thnks for the reply
I believe the issue 4thDan was bringing up was to this effect
take care

John's right. the mute function in the game does work.

do you know if this is for all games on this site?

Are you sure about that John?
-you are saying if you mute a spectator in game your opponent won't see their messages either?