So my work has a benefit for backup care service. I call the service and they find a nanny for me for however many hours or days I need it.

I've used the service for the week now and have had 3 nannies so far. They all have worked out. Today, a new nanny came and she's just not working out. (They other nannies were not available so that's why I got another assigned to me.) She's only been here for less than 2 hours. I'm working from home today and there's just somethings that bother me. Enough that I don't want her coming back. She's supposed to come back tomorrow.

Money isn't the issue. I'll let the service know that she didn't work about and they'll try to find someone else. I am aware that they might not be able to find someone. Although, they do work with placing nannies with only a couple hours notice. If I didn't already have her scheduled, I could call them tonight and ask for a nanny. (Oh, the service is with Bright Horizon Backup care and I'm pleasantly surprised with the nannies I've seen. Almost all of nannies, I would use again.)

I've never fired anyone before.

How do I do it? What's the best way to do this?
I don't want to sound rude so I want to do this as politely as possible. (If that is possible.
Has anyone done this before?