Unfortunately there's no way to "turn off" the mix pot short of removing the pot and soldering in a fixed resistor. But if you set the mix pot fully clockwise (100% wet) rest assured you won't be getting any dry signal from the tape delay once you mix it back in via the controls on your fx loop. 

The preset system might be helpful for this, in that you can save your presets with the mix set to 100% so you can always be sure it's dialed in correctly.