Try using the "Innate" field. It's very useful. If you enter a value in the "Innate" field on the spell, clicking it will not use a spell slot. You'll also see a gray notation on the character's Spell List that serves as a notation. In my games, I'll add 2 Spiritual Weapon listings, one for the initial casting and the other for an existing weapon. For the 2nd, I put "Existing" or somesuch in the Innate field to cue the player. This feature also works great for magic item spells, such as a Wand of Magic Missiles.