The worldmap function constructs map axes for given regions in the world. It specifies the map axes limits based on the regions input. To remove Antarctica while not shifting the region displayed on the axes, first display the full worldmap and then remove the Antarctica section of your plot using logical indexing. An example using the coastlines data is below using the rough cutoff of 63 degrees south:
worldmap('World')
load coastlines
antarcticaregion = coastlat < -63;
plotm(coastlat(~antarcticaregion),coastlon(~antarcticaregion))
