If you want to remove them manually, then you have to go to the excluded client list and remove them. If you want to, you can disable that feature in the advanced tab in the WLAN SSID. Verify the wireless profiles on the client and make sure they are not trying to connect to another ssid that is configured on the WLAN, this will also cause then to be excluded. Unless you only have one ssid configured.
Clients get excluded for different reasons, e.g. too many authentication requests in a given time. This can be caused by a wrong username/password. Did you reenter the credentials on the affected clients?