Zoom your icons instead of standard that will disable landscape mode. Also lock the portrait mode that will stop landscape as well.
Although using the zoom mode will disable landscape mode on the home screens, it will also zoom every app as well because it changes the number of virtual points that iOS is using to render the display.
In standard mode, the 6 Plus has 736x414 points and runs @3x to attain its 2208x1242 rendering resolution, which is then downscaled to 1080p.
In zoomed mode, the 6 Plus adopts the 4.7" 6's 667x375 points, and runs it @3x for a 2001x1125 rendering resolution, which is also then downscaled to 1080p.
As a result, you'll lose all of the UI options unique to the 6 Plus and get 18% less content on the screen. I recommend staying with standard mode and just using the rotation lock as necessary.