In the zocalo it is quite common to be approached by young men, these are the "zocalo boys". They will usually approach a tourist and begin to strike up a conversation. It could even be as simple as "do you speak english?". This allows them to begin to talk and eventually gain the person's trust. This is the way that these people prey on tourists, they will just end up scamming you. I found that the best way to get rid of them was to just respond in a different language other than spanish or english. They will walk away, assuming that they cannot communicate with you.
At all costs, these people should be avoided.