In operaprefs.ini in the preferences folder, set Spellcheck enabled by default=0 under [Users Prefs] (while Opera is closed).
Also, see this page (save it locally and then load it if you're not using Opera 12 for a browser). That will show you how to add opera:config back as the setting is in there.

Just to make it clear for others (since I was really confused about this): You need to add this line to the file. There is no such line as Spellcheck enabled by default=1 in the operafrefs.ini file! Simply copy and paste Spellcheck enabled by default=0 into the operprefs.ini file