def stop_instances(region, instance_tag="my_instance" , instance_state="running"):
	"""
		The function
		1. filters all the instances by tag and instance-state
		2. stops the filtered instances (does not terminate/delete them)
		3. returns the just stopped instances (ids) for future reference
	"""
	c = boto.ec2.connect_to_region(region_name			= region,
								  aws_access_key_id 	= aws_access_key_id,
								  aws_secret_access_key = aws_secret_access_key,
								  )

	stopped_instances = []

	reserves = c.get_all_instances(filters={"tag-value":instance_tag , "instance-state-name":instance_state})
	for r in reserves:
		for instance in r.instances:
			instance.stop()
			print "OK! instance -"+str(instance.id)+"- was stopped. \n"
			# save the just stopped instances
			stopped_instances.append( instance.id )

	return stopped_instances