Camera moves with no FPC input?

I'm using the First Person Controller. WASD and mouse work fine for movement. However, when I change the X,Y axis input for the mouse to a controller (for USB controllers) or button input (IJKL for example), it works but the CAMERA KEEPS MOVING in the direction I pressed even after I let go, unlike the WASD which stops the player when I stop pressing the keys.

This would work for a drunk simulator game, but not my project. What do I need to change to stop this effect? There are other questions like these but no definitive answer with instruction.

Thanks a billion in advance.