Here's how to disable the riffs:
1. Open Sampletank.
2. Select the "PADS" tab.
3. On the left of the screen you will see the "Patterns" section with 3 buttons - one with lines and 3 dots, one up arrow and one down arrow. The first button is the pattern button and is lit (on) by default.
4. Tap the pattern button to turn it off. Now you can play lower notes without triggering riffs.
Unfortunately the riff button is always on by default. The good thing is that it is linked to presets so you can create a preset from scratch with the pattern button off especially for sequencing.