This is strange because a "cold slap" normally would mean something shocking used to stop a behavior or help initiate one. However, if you are "sticking it in someone's face all the time", it is not shocking at all.
A "cold slap" is often done if the person is doing something stupid and does not realize it.
For example, people are talking at a funeral before the priest begins the service. When the priest begins speaking, two people continue talking because they don't realize he has started. You try to interrupt them by telling them that the funeral service is starting, but they do not hear you or just ignore you because they are paying attention to each other. You give one of them a cold slap, which ends their conversation. Suddenly, they realize that the priest is talking, and they apologize for being rude.