ACT! Premium allows you to restrict specific fields to specific users.

How to Manage Field Level Security in ACT!

http://kb.sagesoftwareonline.com/cgi-bin/sagesoftwareonline.cfg/php/enduser/std_adp.php?p_faqid=19185&site_ID=

ACT! does not allow you to restrict specific tabs in ACT! but you can work around this.

1. First, make a copy of the layout for one set of users and another for the other set of users. Remove the tabs accordingly from the desired layout.
   
2. Next, use your Windows file permissions to restrict accesses to specific users for each layout file. On the server, open the databasefiles\Layout folder and right click on each layout. Choose Properties > Security. Define which user has access to the selected layout file.

If you do this, you should also restrict field access since fields are accessible via the Contact List view otherwise.

More information to review: Security Levels in ACT!

http://kb.sagesoftwareonline.com/cgi-bin/sagesoftwareonline.cfg/php/enduser/std_adp.php?p_faqid=15284&site_ID=