What if we could disable some worlds.

If it's just not wanting townies from other worlds showing up in your town, neighborhood traits (like lot traits) may be the easiest solution. You could than choose what kind of sim show up and even how they behave.
For example, there could be an 'only sims living in world' or 'only sims living in neighborhood' trait, a 'only supernatural' or 'no supernaturals' trait, a 'stray' trait for cats and dogs, and a 'tough neighborhood' or 'friendly neighborhood' where sims are more likely to be mean or nice to each other. It would give us a little more control over the sims in our neighborhoods and may be a bit easier to add over blocking off whole worlds.