Does anyone know what happens if you purchase the places where you have to go to recruit more hoods? Does this stop rival gangs being able to recruit?

Hard to say. I don't think it stops them from recruiting there. It certainly does not stop the enemy hoods from arriving and making the attempt to recruit.
I bought a gym and put a prize fight ring behind it. Enemy hoods showed up to recruit every week. Other enemy hoods, from the gang that was aggressive toward me at the time, also showed up to kill the guy running the gym, and the guy running the prizefights.

I find I have to guard my businesses by placing guys inside *and* putting guys outside on the street to deal with any hoods who approach. This prevents the other gangs from figuring out which businesses I own, which in turn prevents the other gangs from sending directed attacks against those businesses.