By default postgrey is enabled in the ispCP postfix configuration. If you don´t want to use postgrey you can disable it this way:
Open your /etc/postfix/main.cf and comment the following line
check_policy_service inet:127.0.0.1:60000,
like this
#check_policy_service inet:127.0.0.1:60000,
Do not forget to restart postfix after this change.
# /etc/init.d/postfix restart
Then you can stop postgrey.
# /etc/init.d/postgrey stop
You should also remove it from the startup scripts.
For further information about greylisting and it's advantages and disadvantages look at Wikipedia.