Interrupting teleop from custom package

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Hi all.

Please I need some help with code I'm writing to interrupt the teleop node on my Turtlebot3. I have the following code:

#! /usr/bin/env python

import rospy
from sensor_msgs.msg import LaserScan
from geometry_msgs.msg import Twist

#defines a callback
def callback(msg):
    # while not rospy.is_shutdown():
    print('==================================')

    # REAR
    print('REAR [0]')
    print msg.ranges[0]

    # RIGHT
    print('RIGHT [90]')
    print msg.ranges[90]

    # FRONT
    print('FRONT [180]')
    print msg.ranges[180]
    if msg.ranges[180] == 0:
        print "STOPPING FRONT MOTION"
        move = Twist()
        move.linear.x = 0.0
        move.angular.z = 0.0
        pub.publish(move)

    # LEFT
    print('LEFT [270]')
    print msg.ranges[270]


if __name__ == '__main__':
    try:
        rospy.init_node('obstacle_avoidance')
        pub = rospy.Publisher('/cmd_vel', Twist, queue_size=10)
        rospy.Subscriber('/scan', LaserScan, callback)
        rospy.spin()
    except rospy.ROSInterruptException:
        pass

In the section of code commented # FRONT, the robot is in motion using the teleoperation package and I am attempting to stop the robot cold in it's tracks. However, what happens is the wheel splutter i:e stop and continues in very quick succession.

Can anyone help point out my error, please?