in the BO under utilities > templates >system > Back Office there is a template called "menu.xml".
If you look at the xml you will see at the end of some xml nodes a tag like: roles="Manager"
You can add this tag to the role which you want your client to have access to.
What I do is to set the menu items that I don't want the client to have access to to : roles="Administrator" and the menu items they do have access to to roles="Manager" because I don't give my clients to Admin functions of the site.
Be very cereful when changing the xml, an error will cause the BO menu system not to display and then you'll have a problem ;-)
Copy the xml to vs and change it there then copy it back.