To turn off aftertouch on specific channel strips in MainStage, you have to select the midi data type and then choose to filter it out. It's also been renamed to "channel pressure."
What Gear do I need?
Troubleshooting Sunday Guitar
My instrument/controller isn't connecting
My Song-Specific Patch is Different from the Demo
Customizing Your Song-Specific Patch
Sunday Keys Version 2
Using the Tonic Pad Player
Getting Started with Sunday Keys for MainStage
Troubleshooting Sunday Keys for Ableton
Getting started with Sunday Keys for Ableton