You can use the access pages manager to Add / Remove security access restriction to pages in your site. or you can remove it from individual pages using the server behaviors panel. open the page you want to remove access from, open the server behaviors panel, select the security assist access restriction server behavior and click the minus button to remove.
The strength of security assist depends on the strength of the passwords you use. The most common method a hacker would use for trying to gain access to your pages would be a brute force attach where a script keeps trying your log in page and trying to guess the username / password combo.
Using strong password with a combination of uppercase, and lowercase letters, numbers and special characters will prevent a brute force attack.