What do I say to cancel a housekeeper due in three weeks? (More inside)

So I needed someone to help me out and she was new . She didn’t clean the microwave at all, left lots of clean dishes, etc. in the sink, vacuumed only half of a bedroom and other things. I want to call her so she won’t come but what excuse do I give? Note: I will not tell her she was unsatisfactory or left things out. I am also missing a nice ring but cannot prove she took it and I think she’s innocent of this.