KActors (or any actor with it's physics set to PHYS_RigidBody) can only have their velocity directly modified by accessing the primitive component being used to calculate the rigid body physics.
StaticMeshComponents are typically used to calculate rigid body physics, and are (eventually) descended from PrimitiveComponent, which has the following function:
For example, the following code would cause a KActor stop moving.
* Set the linear velocity of the rigid body physics of this PrimitiveComponent. If no rigid-body physics is active, will do nothing.
* In the case of a SkeletalMeshComponent will affect all bones.
* This should be used cautiously - it may be better to use AddForce or AddImpulse.
* @param NewVel New linear velocity to apply to physics.
* @param bAddToCurrent If true, NewVel is added to the existing velocity of the body.
native final function SetRBLinearVelocity(vector NewVel, optional bool bAddToCurrent);
The PrimitiveComponent class has a number of functions that pertain to accessing or manipulating rigid body physics, most of them can be easily identified by looking for the letters RB (for Rigid Body) in the function name.