For starters: See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a detailed description of how to set this up.
This describes a method you can use to limit a user's ability to view/access records on a record by record basis.
Get ( AccountName ) or Get ( AccountPrivilegeSetname ) can be used with this method to limit a user's or group of users' access on a record by record basis.
Note that finds performed by the user or by a script will automatically omit "no access" records from a user's found set.
And when playing with Manage | Security, it's good to do two things:
Save a Backup copy of your file before changing security settings so that you can't accidentally lock yourself out of the file.
Add a button that uses Re-Login to open up the password dialog when you click the button--this allows you to change logins quickly and easily while testing your security settings and associated scripts.