You could rename / remove mspaint.exe from C:\Windows\System32 When you open a file extension linked to MSPaint it will throw an error message and you will be asked to chose a new default program.
Or you could just link the file extensions you want to your editor of choice: right-click file, select Open With and tick 'always open with this program') mspaint.exe is like 5MB so it's not taking up much space.