In your case, I suggest using another router functionality instead of traffic rules. Navigate Service > Web Filter and create a whitelist. In this whitelist write only one address that you want to access. Every other address will be blocked.
If you want to keep using traffic rules you just need to create a new rule, using a custom source address. Keep in mind that traffic rules work in order from top to bottom. Make your Allow rule the highest priority by dragging it to the top.