There's a grey tab with a page icon on it at the top right of the page. Click on that to open a "drawer" of pages at the top of the screen. There ought to be an icon representing each of the pages in the campaign, including any one you want to remove the grid from. Click on the cogwheel icon to the upper left of the icon of a page of your choice. A dialogue box will open. Somewhere in the lower part of that there ought to be a check-box with a check in it, labelled "enable grid". Click on the check box to remove the check. Click on "Okay". Click on the grey tab with the page icon to close the pages drawer.
I can't explain which it should have taken so long to address the issue that a lot of people don't want 70 pixels to represent five feet on every (or in a significant number of cases any) map.