For now to beat the ai is pretty easy: you just rent any available office/retail until all your rivals dont have the opportunity to expand and they go banckrupt after some time.
This could be fixed/tuned in a way to either make renting more expensive for player and/or make the ai more prone to expansion.
The best and most realistic solution would be to add many rentable floors for one building. That way it would be really hard to squeeze out a rival from a district.You would have to rent all the foors in the building or buy the building entirely.