Under Master > Touch, mine is set to "Med-2" - this is the closest thing to a global setting for aftertouch. But, this setting controls both aftertouch+velocity. The Soft-Med-Firm-Hard categories refer to velocity response (how hard you play the keys). The 1-2-3-4 of each setting refers to the aftertouch sensitivity. Set it to a 3 or 4 to make aftertouch difficult to invoke.
