How do you deal with unsolicited opinions and over bearing in laws?

How do you guys approach the topic of unsolicited opinions and too involved in laws? I know I'm going to need to have a conversation with them soon, but have NO idea how to approach it without coming off rude, yet still very very firm. 

My MIL thinks her and her mother get to choose everything for baby, including his nursery, without even so much as asking what I would like or my opinion. She thinks she can dictate how my husband and I spend our money, and yells at him every time she finds out we have purchased something for our son. She thinks ever since we gave her a spare key for emergencies that means she can come over whenever she likes to "help with baby". She keeps implying she will be staying with us immediately after birth. Unwanted opinions on his name, etc etc etc... 

I understand that they're excited but they seem to forget that WE are excited too and want to make our own decisions, if not at least be involved in making them. We wouldn't have gotten pregnant if we could not afford a child and I do not expect anyone else but my SO and I to provide for him. Gifts and help are welcomed but I cannot even purchase a mobile that I love without getting an upset phone call "but grandma wanted to pick that out! You shouldn't have bought it!!" 

I'm trying to stay calm and cool but add in some preggo hormones, I'm starting to get really frustrated and worried if they're like this now, what will they be like when he is here? How do I nip this in the bud without them hating me?