If you have a user who you wish not to allow access to your site, you can do this without deleting their account.

Step 1: Create a User Group

Give the User Group a name and put in the Administrative Description that "Users in this group may not log in."

Step 2: Send a Message to Support

Send a message to support requesting the permission be applied to the User Group. 

Note: This does not require the Advanced Permissions module.

Once the permission is in place, the group will now block any members within it from logging in.

Step 3: Put the User in the Group

Placing a user in the group will block their ability to log in.  Removing them from the group restores their access.

From the user's profile, press Edit and select the group from the 'Select User Group' dropdown.

To remove them from the group, press the X next to the group.

Users placed in the 'Deny Login' group see this message when attempting to log in: