Which version are you playing on?
On the live version (0.9.0.13), you can edit the max population. There's a button on the top left of the screen that lets you do so.
On the beta version (0.10.0.0+), you need to middle mouse click the human population (should be on the top middle stat bar) and set a max there. Alternatively, you can disable the bedroom function for the home. That'll keep them from procreating. But, that's on a per-household level and that might be tedious.