After you've configured the application and allowed it to scan your network, the main screen will display a table with all the devices that currently are or have been connected to your hotspot including their MAC addresses, IP addresses and computer names. The first column of the table is the Block one and all you have to do is to click on the box corresponding to the device you want to stop. Unfortunately, as far as I know, this tool can't actually block "connections" just "devices", so if the intruders change their MAC address, you're going to have to block them again.
For an added lair security you can copy the MAC address of the unwanted device and blacklist it on your router's settings page. Lastly, if you're a more advanced user, you can use a different tool called Wireshark, which is actually capable of blocking connections.
Check the Wireshark's documentation page to learn how to properly capture, analyze and perform different network operations: https://www.wireshark.org/docs/wsug_html/