Press the Clear button above your Speed Brake Lever, it should be lit up, alternately map your side stick properly so the AP Cancel switch works as planned, 1st Press Cancel AP, 2nd Press Cancel Alarm.
As with any other AC in the the sim... you have adjusted your dead zones and sensitivity to suit your flight stick haven't you?
I found "-4" good for sensitivity on the Thrustmaster TCA Stick, sorts out the madness a good bit!