first, they need to get romantically involved, then married, then they will poop out babies soon enough, unless it's a gay couple.
To encourage this, give your dwarves lots of free time to socialize, preferably in a tiny meeting area so they huddle closely.
It's not guarantueed to get a breeding couple going from the starting 7, but usually there's at least one couple formed eventually, in my experience with 7 dwarves restricted playthroughs.
If you want to know if it's possible to mod your dwarves in such way that they will poop out babies like wildlife does, without marriage, you have more chance of finding an answer in the modding section. Not sure if that's even possible but you never know.
EDIT: oh sry, misread your question.
It is not possible to restrict your fort to the starting 7. You will always get at least one or two migrant waves. You can also not restrict your population to 7, because 10 is the minimum possible population to stop immigration. So until you have 3 kids from your starting 7, which will add up to 10, you will keep getting migrant waves.
So to stick with the original 7, you'll have to 'dispose of' those migrant waves, until you have the starting 7 + 3 kids, and the outpost liaison has reported back to the mountainhomes that your population cap has been reached.
in D_INIT you should set POPULATION CAP to 10. (You can set it to 7, but the game engine will see this as 10 as well) This will still allow your dwarves to have babies. STRICT POPULATION CAP controls the cap after which they will no longer make babies.