I reported a bug about this a while ago, and got a response today.
"default search engines cannot be removed but we let our user to create and use their own search engines."

It appears they have no intention of changing this. I've already switched browsers. This is just insane that they are forcing these search engines. I'm curious exactly how much was required for them to sell out their users. The old Opera has been dead for quite a while; I guess these kinds of things should just be expected now...

The Opera 31 update wiped out my custom search engines. I'm also unable to edit/remove "default_partner_content.json" file now; Opera shows an error about invalid file and refuses to open...

Is there a workaround/fix for this yet? I do not want the forced default search engines, and want to use the letters they steal for something useful. I specifically switch from Chrome to Opera to get away from this kind of ass-hatery; I really don't want to have to switch browsers again.