Stopping Coroutines

StartCoroutine returns a Coroutine, a handle to the coroutine it started. Keep it in a field, and StopCoroutine(handle) stops that coroutine where it is paused: the lines after its yield never run. StopAllCoroutines() stops every coroutine the script started. Space switches this alarm on and off:

Coroutine beeping;

void Update()
{
    if (Input.GetKeyDown(KeyCode.Space))
    {
        if (beeping == null)
        {
            beeping = StartCoroutine(Beep());
        }
        else
        {
            StopCoroutine(beeping);
            beeping = null;
        }
    }
}

IEnumerator Beep()
{
    while (true)
    {
        Debug.Log("Beep");
        yield return new WaitForSeconds(1f);
    }
}

The field does not empty itself when the coroutine stops, so the alarm sets it back to null, and the next Space knows nothing is running. Two ways that look right do not work. StopCoroutine(Beep()) calls Beep() again, which makes a new, unstarted coroutine, and stops that one, so the alarm keeps beeping. And enabled = false switches off the script's Update(), but its coroutines carry on. A coroutine ends when it is stopped through its handle or by StopAllCoroutines(), when it reaches its last line, or when its GameObject is deactivated with SetActive(false) or destroyed.