Be sure to check the input selection on the other tracks. You are able to select from all the audio inputs the computer will see..... it's built in mic, anything plugged in by USB that has audio options to it, and NONE... if you select none.... the track should not accept any input at all.
I use a Focusrite interface as do a few others here... I love it. If it dies, I will likely replace it with another Focusrite. With the Focusrite, there is no need for a mixer..... just plug in and go.
Check the track input selections. Some sound blaster cards (built in or "from the factory") have a setting that should be adjusted to prevent that. I've seen Greg mention that.
All input and output monitoring should be through the USB card/interface.