There's no bus mute function
per se. Here are two options.
Instant on and instant off, like a "real" mute button: Insert the Sonitus gate as a bus FX. Set threshold to 0.0 and depth to -infinity. Write-enable gate automation. Click on the Output parameter to choose "Sidechain" (audio unmuted) or "Audio" (audio muted). Just don't exceed the noise gate threshold (i.e., keep the bus input gain under control) or the muted signal will come through.
Pseudo-mute using level automation: Suppose you want to alternate your bus fader between -infinity and -1.6. Set the bus fader to -1.6. Right-click on it and choose
Value > Set Snap-To = Current. To mute, pull the bus fader all the way down. To snap back immediately to -1.6, double-click on the fader.