The best thing is probably to play around with the Google Styled Maps Wizard. There’s also a link to it in the MapPress settings:
Try clicking on ‘points of interest’ and setting visibility to ‘off’. I think that’s what the sample JSON does. Then you can zoom in on the location and see if that works. If you get it working the way you like you can click ‘show json’ and copy the JSON into MapPress.
I doubt that another plugin is doing anything… but if the styled maps wizard doesn’t work for you please give an example with a URL.