A free app for Android, by PixelITC.
This is a powerful and easy-to-use tool that allows you to view all the devices connected to your Wi-Fi network, identify the user, and block the connection. This app is based on the official Android API and the functionality is completely free.
This tool will allow you to block any user, not only the one using your Wi-Fi, which will ensure the security of your network. You can block all the users of your Wi-Fi network, regardless of whether they are connected to your network.