The process some others have described is called binding. You want to bind qBit to your VPN. qBit will only work if your VPN is active then. Close the VPN (while uploading/downloading) and qBit automatically loses it’s connection. This is above and beyond a “kill-switch,” which is notoriously unreliable. Most decent VPNs have a built in kill switch, but you should not solely rely on it.