If you have setup a transparent bridge and all clients are station-bridge then they will all see each other.
Disabling Default Forwarding on the AP/Bridge will give some L2 isolation from clients but the AP/Bridge will see all.
If you wish to seperate the customers then you may be better with a routed solution and use EoIP or VPLS L2Tunnel between endpoints.
WDS could be used but it is not very efficient on Wireless N. See http://forum.mikrotik.com/t/what-is-wds-limitaiton-with-wireless-n/30429/1