only way to do that is to delete the file that holds the annotations. You can do that by closing MT, then deleting the file named something like "your@emailaddress_ChartDrawings.bin" in the User "Roaming" directrory. If you go to MT HELP / About.. page, you can click on the "Roaming" link there to open it, or go to:
C:\Users\<Your Windows Login Here>\AppData\Roaming\2GK\Medved Trader