joy_force probably isn't the cause of the problem it more than likely lies somewhere else in your code.

simple test: create a level that only rotates a simple object and see if the problem continues
Code:
#include <acknex.h>
#include <default.c>

action the_player(){
	while(me){
		vec_to_angle(my.pan, vector(sign(joy_force.x),sign(joy_force.y),0));
		wait(1);
	}
}

void main(){
	wait(1);
	level_load(NULL);
	ent_create(CUBE_MDL, vector(150, 0, 0), the_player);
}

if you still have the problem display joy_force.x and .y to determine if they're not resetting and what their values are
Code:
...
DEBUG_VAR(joy_force.x, 20);
DEBUG_VAR(joy_force.x, 20);


if these are showing there's still problems try it with a different joypad. if these aren't the problem post some more of your code as the problem is elsewhere