Hi cbentley

Its really simple just go to
C:\Users\"username"\AppData\Roaming\Opera Software\Opera Stable\
and open your "Local State" file with e.g. notepad and add: "magnet": false, to the "excluded_schemes" section
Be sure to flag: false

Like this:
"protocol_handler": {
"excluded_schemes": {
"afp": true,
"magnet": false,
"data": true,
"disk": true,
"disks": true,

tested and wrking on opera 18 and 19
Hope it works for ya