Defending blinds is tricky, especially at no-limit play. Your position is horrible, and doesn't get better, unlike stud. That you get a "discount" to call is pretty much irrelevant, and serves to suck in the fish.
Unless you are certain you're up against a "thief" on the button, or in the cutoff or hijack, I'd throw away the hands you mention here. If A7-o is garbage UTG, it's garbage in the BB, and even worse since, at least, the UTG has position on the SB and BB.
Playing out of the blinds is the worst situation to be in since it's a sure fire losing proposition. You will often be better off discarding when in the blinds than calling unless you're holding a strong hand.
In limit play, you can defend more vigorously as position isn't so important in limit play as it is in no-limit play. Being limit, your whole stack isn't at risk when you play OOP, not as costly to make mistakes, as these cost you only a few bets. Still, it just doesn't pay to try to prevent every act of larceny when you're in the blinds.Even in limit play, you're better off being in position, as it's easier to get extra bets in when you're ahead, and to save bets when you believe you're behind.
It's bad to be OOP, but there's not much you can do about it: it's the nature of the game.