There is a button called 'Force GAL sync' that can be found on zcb logging control (type zcologctl.exe in command prompt). Pressing on this button will initiate GAL sync immediately. Please note that it is important for BB controller and BB manager to be running before pressing the button as GAL sync depends on those two processes.
Alternative solution to the problem would be to open up 'BlackBerryServer', 'BlackBerryManager' profiles with Outlook and manually delete unwanted contacts from the GAL.
Please feel free to look for other solutions in zimbra forum dedicated to BES:
Zimbra Connector for BlackBerry - Zimbra - Forums