By far the quickest way is this:
In the mix recall module drop down the menu and pick "Reset Mix...".
By default this will delete all envelopes and fx in the project and "zero the board"
However you can completely customize what reset does by opening the mix recall preferences (by default it will open the mix recall preferences when you choose reset)
Please note that mix recall operations are not undoable so save your project before you reset the mix.