You probably have to store all the admins that are currently on the server at the time the map changes and then when the map starts, check to see if they are in that list. If they are, don't print their name.
A less reliable but easier alternative is to not display any names within the first X seconds of the map start. This means that the majority of the time, the admins that were already on the server at the end of the last map will not get displayed. Only the admins that join after X seconds after map start will be displayed.