I would like to know if there is any way to disable Spotlight Search, even when an user goes to Settings -> Spotlight Search, and change the options there.

That's mainly because anyone could still search in Spotlight only by going to Settings -> Spotlight and enabling the search on the web. I do know that you can set it so you can apply filters, but they are not successfull at what I need. (Yes, I know also that Safari can be restricted under "Restrictions", but I am only talking about Spotlight)

I would need something like "Spotlight Search" under "Restrictions", so I can turn it off completely, so any user can't enable the Internet search again.

That's like a little bug for anyone who want to be fully protected for mature/adult content. (I want to have my own filter, not what Apple says that is good)

PS. Currently I am with iOS 10.X in an iPhone SE. There's a video from youtube here that explains how to disable Settings app. That would be great, but that workaround only work in older versions of iOS.