You'll never be completely stable, but there are a few tricks.
You can "demolish" houses but not have any builders. This will cause the family to be kicked out (or if its empty it will stay empty). If you have a few empty houses, you can close them. This will slow your growth, as new couples will be unable to use them to move in and produce children. When your growth is too low, reopen the houses by stopping the demolish.
In general, you want to keep the population about 70-15-15% adults, students, children. That way you have enough children to replace workers. If you have this ratio maintaining a stable population should be more natural and less work.
An important note is your build up to the 30-50 range will make a difference in how stable your pop is. Increase your pop very slowly, trying to stay at or below 15% children. A huge growth period will cause all your children to be the same age, causing a mass die off later, causing a mass birth later, causing population instability.
If I'm increasing my housing (you actually need more houses than you'd think for an age-spread town) I pause it right before it finishes and wait until I need it for reproduction before I let them build last few percent.
Another hint is to use the town hall graphs! You'll see (and understand) the natural ebb and flow of your pop. If you have a population bust or boom try to find out why so you don't repeat the mistake (too many houses, too closely aged pop). Two important non-graph numbers are the number of families and the number of houses.
Things I don't know:
- What the minimum stable town size is
- How tight of a control you can achieve
To get the 200 years achievement I kept my pop under 100 for the first 125 years or so. 30-50 seems about as low... but I believe it should be possible. It might need to be 30-60 range though.