If you don't want your PIL's help then you need to communicate that clearly but politely to them. The tricky part is that your DH seems to be happy with their help so they are probably receiving mixed messages.
I had a different but similar situation when I married DH. His parents had bought him a flat and DFIL had decorated it and put in the kitchen and bathrooms before we met (this is normal practice in DH's country). The problem is that I hated it and did a pretty bad job of showing it. DFIL had poured his heart and soul into the work and was so proud, and I stamped on it. If he weren't such a forgiving person I don't think we would have a good relationship now and I cringe every time I remember my behaviour. I was bloody awful.
Now we want to sell that flat and buy a new one, and DFIL is threatening to decorate it for us. He loves marble, gold everything, giant Chinese vases and draped gauzy fabrics which can't be washed and collect dust, I like simple things and nice wood. DMIL just gave him a
look and he stopped talking about it. 
In the meantime we did buy a holiday flat that was ready decorated but needed to be furnished and PILs furnished and equipped it to their tastes. I found it quite liberating to relinquish control and be happy that they were choosing things to their tastes. They are so proud of it and take people there all the time. They also made better choices than I would have done, even if I'm not super keen on the look.
What I am trying to say with all this rambling is that although decorating your first house is very important to you and understandably so, your relationship with your PILs is more important in the long run and so if you can't make them understand your point of view it might be worth just deciding to let it go and then have lots of stuff mysteriously break.
Do make sure of a common position with DH though, because this is the key.