If you totally want to disable velocity, you'll need to check in a few different places. However, most of the time it'll be pretty easy:
(1) Push the Envelope button until all three envelope lights are lit
(2) Push the Shift button
(3) Turn the "Velocity" knob to zero
Many patches implement velocity control by setting velocity response on the envelope(s), so that will work most of the time.
If you're still getting some velocity response after that, you'll need to dive a little deeper:
(A) If there's a Velocity destination set in the Misc Mod section, make sure its amount is zero
(B) Check the Modulation slots for one whose source is Velocity, and set its amount to zero