How to Force the Usage of sFTP on my Dedicated Server
Since the regular FTP service and sFTP work on different ports, all you have to do is to block port 21 (regular FTP) for incoming connections.
To do this login to your Dedicated server as root via SSH and type the following command:
iptables -I INPUT -p tcp --dport 21 -j DROP
Note that you will have to do this each time you restart your firewall.