Actually Ash had perhaps the best. Just be busy. Happily apologetic, but busy.
Oh! I'm so sorry, but I'm all booked. I know...uh huh...right? hmmmm, let me look: wow, all busy that day as well. I know! so sorry, but I am happy to be booked though. uh huh...I know...uh huh
Ask if she would like to be on a cancellation list for any particular day. If you have an opening you can call her and see if she makes it down. Then she will NEVER even know you fired her eh?