To decrease the transmit power of your D-Link DSL 2730U
Make sure your router/modem is connected through a LAN port. Now follow these steps:
Open https://192.168.1.1 from your browser and log in to your router (default username and password is admin and admin respectively)
Now go to Wireless > Advanced now on the bottom you will see Transmit Power which is set to 100% by default. You can change it to different levels i.e. 20%, 40%, 60%, 80%. You have to try each value until you get the desired Wi-Fi range. After setting Transmit Power click on Apply/Save.
By reducing the transmission power your WiFi radius will be reduced. It is highly recommended to set a secure password for your Wi-Fi Network. Make sure to set a password which should be at least of 64-bit length. In a 64-bit password you can either set a 10 digit number or a 5 digit ASCII code.
Update[6-August-2016]:The above method is for DSL-2730U U1.