You cannot truly disable the ‘New’ button per se however, you can change the functionality so that it does not have the function of creating a new file.  You can change it so that when you hit the button it will start recording instead of creating a new file.  This will also give the slide switch additional functionality. 

Go to the file menu and choose ‘Device’ > ‘Programmable Buttons’.

To the right hand side you will see ‘New and Slide Switch functions’  Choose Type B or Type C to change the new switch to ‘Record’.  Note:  There will be changes to you slide switch functionality when you make this choice.  When finished, hit the update button to apply changes.