The code is a bit messy, but I think you'll want to set "mouseClicked" to false when you right-click. So in the Input.GetMouseButton(1) if statement:
if (Input.GetMouseButton(1)) {
Debug.Log ("Pressed secondary button.");
rightClicked = true;
mouseClicked = false;
}
If you then want to fire the bullet again, you'll want to add the reverse in the left-click if statement. So you'll want to set "rightClicked" to false there.
At the moment it's continuously aiming because you're never telling it to stop doing so.
Edit:
As you said in your comment, you want to only be able to click the left mouse button once, then ignore it.
That can be achieved by adding a new boolean to check whether or not the left mouse button can be clicked. As displayed here:
private bool canClick = true;
if (canClick) {
if (Input.GetMouseButtonDown (0)) {
Debug.Log ("Left Mouse Button was pressed");
moveSpeed = 0f;
if (sr.sprite == sp1) { // if the spriteRenderer sprite = sprite1 then change to sprite2
sr.sprite = sp2;
}
mouseClicked = true; //register that the mouse has been clicked and the sprite is changed to sprite2
rightClicked = false;
canClick = false;
}
}