Limit Salesman Access
If you would like to block salespeople from seeing orders, invoices, and customers other than their own, you can enable the Limit Salesman Access add-on.
After you have enabled this add-on go to Admin -> Billing -> Company Options -> General -> General and turn on the option labeled Block Salespeople From Seeing Orders, Invoices, Customers Other Than Own.
In order for the system to know which user to block info from, you will need to link together the salesperson and the associated user account. Go to Admin -> System -> Users, select the user, click the Additional link in the General section. Select the salesperson to link the user to with the User Is This Salesperson select box. Click Save. The user will now only be able to see customers, quotes, orders, and invoices that are linked to their salesperson account.