Divide the input by a constant before sending the value to the motors. I'm not too familiar with FTC, but in RobotC for Vex, it would look like something like this:
Code:
motor[port1] = vexRT[Ch3] / 1.5;
A better solution would probably be to re-gear the robot for the desired speed. In changing the maximum speed in code, you cannot use the maximum power of the motors.