Okay here I am assuming that there will be few people in the organization who will be entering the contacts and then assign to somebody else.

1. Change System Default Access - This needs to be done because by default everybody can see all the contacts but they will not be able to modify it. To restrict this:
   • Login as an Organization Administrator
   • Click on the Setup at the toolbar and click on Manage Organization Roles and Privileges under Organization Management
   • Click on the Manage Application Access Defaults (the last link)
   • Now go to the row for Contacts and click on the Edit Icon (on the right)
   • Remove the Read Permission (so it should now say, No Read, No Write, No Append and No Delete)
2. Now make sure that the people who are entering the contacts should have privilege to create and assign contacts (by default this is granted) but can be changed from the Manage Member Roles (under Setup >> Manage Organization Roles and Privileges)

let me know if you need more information on this.

BTW: We have a blog article that you might enjoy: http://blog.officeclip.com/post/2009/12/14/Additional-layer-of-Security-for-web-based-applications-in-OfficeClip.aspx.