Why not talk to her instead of deciding for her and issuing ultimatums?
Has the teacher said that she's making slow progress or is that your estimation?
It seems as if you may both have different expectations about the lessons, maybe she just wants to go because she enjoys it. Maybe she thinks that she is doing enough practice - depends too on what the rest of her week is like - does she have a lot of homework etc?
I'm not sure though why you want her to stop - it seems almost as if it's because she's not making the progress that you think / would like her to make, therefore, in your view, it's a waste of time and money.
If it's for financial reasons, then explain that you think you're making the commitment (time and money) but she's not and see what she says. She's only 11, so perhaps she doesn't realise how you see it. If that's the case, then see if you can work out some kind of a compromise as HL mentions above.