Some cats are very motherly. If your planning on finding homes for the kittens then it's best to take them away from her permanently as long as they are eating kitten food. Keep them in a room that she doesn't have access to. If you plan on keeping them she should be able to continue nursing until she feels it's the right time to stop. If you plan on keeping one of the kittens let that kitten stay with her and separate the others. My momma cat had two kittens that I decided to keep, she nursed them for five months then she felt it was time to stop and weened them on her own. My vet's advice was to let her nurse, she'll stop when she wants to, and she did. She's a very good momma and even took in a 7 day old kitten that had lost it's mother. She nursed him back to health and took care of him like he was her own. It sounds like you have a momma kitty just like her.

 It's unusual... But unless she's suffering nutritionally, there's no reason to worry about it... When the go to their new homes, they'll eat on their own.

 Its ok they will back off  slowly she will say no when its time

 I think it's cute that the mother wants to do that, there is nothing wrong with it, you just have a motherly mommy on your hands!

 Well id say no problem if you were keeping them all but assuming your not i think its time to find loving homes for them

 It isn't that she won't let them stop nursing, a kitten can persuade it's mother to let it nurse by nudging at the mother's underside, whereas she will lay down and let them eat.
It's perfectly fine that they nurse if you're going to keep them or not, they can only benefit from the mother's milk and be healthy from it. Hope this helps, good luck. Pet the kitties for me, I love them.

 OK, so what you need to do is take the cat to a vet along with her kittens so they can get checked out.
But before you do this, you need to check the mother cat for any stress marks.(remember that the stress marks are on the head and under the fur!)