Kittens won't stop nursing!

I've got a litter of kitten and a mother cat. There are four kittens and they're outside cats. The kittens are around 5-7 months old (I forgot when they were born) and they won't stop nursing! I wasn't too worried about it before because they eat solid foot and nurse. Sometimes at the same time. But now the mother cat is pregnant again and I'm worried about them nursing while she's got other kittens, I figure it will be a problem. She growls at them sometimes, but nothing that has gotten them to stop. I don't know if she'll make them stop when she has the other kittens, but just in case she doesn't is there anything I can do to get the kittens to stop nursing? I've looked around and I haven't found anything as of yet.

Yes it is a problem.  It is very hard on her to be nursing these big half grown kittens while also gestating a new litter.  

  You can   try putting Bitter Apple spray  on her nipples to discourage them and see if that works for now.    

If that does not work   you're going to need to keep the mother separate from the older kittens starting now..   You will have to do that anyway when she has the new litter because the newborns should not have the big kittens competing for milk -- and you can't put on any bad tasting stuff to stop the newborns from nursing!     but you need to start it now so the adolescent kittens are not taking up nutrition that the mom needs for herself and the fetuses in her. 

Are the 5-7 month old kittens fixed?  If not that needs to happen NOW.  or you are going to have them inbreeding.   Some kittens reach puberty even at 4 months.  and if these haven't yet,  it could happen any time.  I know of male kittens who have sired litters at 5 months.  Don't count on being able to know for sure when the girls go in heat.  The first heat(s)  can be more subtle and often humans don't recognize it.  but tomcats do!    This is how people end up with 50 cats!  

I don't know how far along she is but one  possibility is spay/ abort.    It is not a pleasant thought but neither is the fact that so many already born kittens  are killed at pounds for being homeless.   Cats don't have human religious beliefs,  and they don't mate with the conscious idea of wanting to have kittens -- it's just instinct for them.    Some vets won't do it,,  some will do it up to a certain point in the pregnancy,  and some ( especially shelter vets who see the problem of so many homeless cats)  would even do it late.  though I myself would not   do that unless the mother cat were in very poor health.

If you do continue the pregnancy,   please keep her inside and separated from any male cats,  Cats can get pregnant even less than a week after giving birth.  which would be  bad for her and the kittens.

      Then have her spayed when the new kittens are 8 or 9 weeks old.

When I say keep her separate from males that is including any males in her last litter ( which you need to separate anyway because of the nursing... but also the males might mate with their mother ---  and even when you get them neutered the boys can still be fertile up to 7 weeks after neutering! ) 

I dont know where you are located but many places have low cost spay/ neuter clinics now.  and even if you had to drive a few hours it would be worth it to have that done.   

If you're in the USA,  there is an organization called Spay-USA that has a hotline number you can phone.  http://www.spayusa.org/  

Also the ASPCA has a database on their site of low-cost spay/ neuter clinics.  http://www.aspca.org/pet-care/spayneuter

If you are somewhere else,  try searching online for low cost spay  and your location.   or talk to some cat resscues  / shelters in the area and ask if they know of any .