One of the steps to tighten security for an OpenCart 1.5 store would be to require new customer approval before they can log in.  The following tutorial walks you through enabling the customer approval required setting in the Administration Dashboard.

How to Enable Customer Approval Required in the OpenCart 1.5 Administration Dashboard

1. Login to the Administration Dashboard as an Administrator
2. Hover over the menu bar where it says SYSTEM and then click on SETTINGS in the drop-down menu
3. Select the STORE that you wish to edit and then click on EDIT on the far right to proceed
4. Click on the TAB that says OPTION, then scroll down until you see the option labeled APPROVE NEW CUSTOMERS as shown below:
   
   
   
   
5. Select YES to enable the option, or NO to disable it.  Note that with the option on, new users would require approval before being able to login to the site. 
6. Select SAVE in the upper right hand corner to save your selection.

The Approve New Customers option in the Settings of the OpenCart 1.5 Administration Dashboard is another way to increase security on your store so that no one can login to the store without first being approved.