Growl is controlled completely independently in its own preference pane (in the System Preferences application).
Synergy's Floater is controlled in the Synergy Preferences, in the "General" tab (the first one you see when you open the preferences).
There you'll find a checkbox to turn it on and off, as well as checkboxes to control what information gets displayed.
If you think there's a glitch in the Floater (ie it's missing information) and you're sure the checkboxes are correctly set, I'd recommend that you try stopping Synergy, moving your preferences file out of the way (from ~/Library/Preferences/org.wincent.Synergy.plist) and see if the problem goes away. (Corrupt prefs files are the most common cause of odd behaviour.)
I originally thought you were talking about the Floater, but I now think you are talking about the notification bezels that appear in the middle of the screen and which look like the bezels you see when you use Mac OS X's built-in brightness and volume function keys.
If that's what you want to turn off, the checkbox you need to hit is the one which says "Extra visual feedback for next, previous etc". That'll turn off all such bezels.
Funnily enough, digging through the source code I can see from comments like this one...
// we always show the rating feedback floater, regardless of prefs
... that this behavior was actually intentional.
I can't really remember what my reasons were for taking this stand at the time, but I think there's probably a case for making this bezel fall in line with the other ones.
Other bezels which are unconditionally shown are ones like iTunes volume up/down bezel, but that one is not so much of a problem seeing as it doesn't seem like a duplicate notification for Growl users.
I'm just going to leave this one sit for a day or two while I think about what the best course of action to take here is: either sweeping changes to all such bezels, no changes, separate options to control each type of notification, or some other "in between" approach.
I'm thinking that the best thing to do will be to add another checkbox. So we would have the existing "Extra visual feedback for play, pause, next and previous track actions" checkbox, plus a new one "Extra visual feedback for other hot keys".
Thank you! It's the niggling little inconsistencies that just bug the heck out of me, you know? ;-) Appreciate you taking the time to fix... And you have good commenting style. Document what you did. Beats the "// fix this hack before a client sees it" comment that seems to show up at the worst possible times...