Don’t want to update to OS X Yosemite? Tired of dismissing this notification every few days?
You can hide/suppress this notification with a simple command using NCutil.py, although it’s not fully-supported by Apple, so you can try this at your own risk.
The above command removes all the hidden notifications that you cannot adjust from the GUI–things like battery notifications, iCloud notifications, USB notifications, etc. So it is sort of like using a sledgehammer to remove the Yosemite notification.
If you prefer something a little more precise, you can try:
NCutil.py --alert-style none _SYSTEM_CENTER_:com.apple.storeagent NCutil.py --alert-style none _SYSTEM_CENTER_:com.apple.noticeboard
You can also use NCutil.py to adjust any of these settings pictured below from the command line.
Let me know how it works for you!