If you are only a moderator then you cannot ban users unless a mod has been installed to allow you to do so..
You need to access the admin control panel and go to the User Admin section and Ban control..
If the owner is unsure how to give you these rights make sure you suggest that they ask here
Moderator Control Panel
It's an excellent MOD... I installed it and integrated with Smilies Upload MOD... now moderators can upload smilies without having a FTP access... moreover they can ban users, delete them... it's very useful!!!
Patience of a Saint :: Memory of a Goldfish.