Hi! So sorry for the late reply!
The GetCharacterMovement() function is defined in the ACharacter class, so if you write your code in the YourCharacter.cpp, then you could use:
GetCharacterMovement()->StopMovementImmediately();
The StopMovementImmediately() function stops the current movement instantly and zeroes out the velocity.
If that wouldn’t work for you, there are other functions defined in the movement component that might else work, such as:
GetCharacterMovement()->StopActiveMovement();
GetCharacterMovement()->DisableMovement();
Hope this helps!