Hide "Add to Opera"
There is any way to hide the button "Add to Opera" on Speed Dial?
The button doesn't say add to Opera. You must be just referring to the + (inside the square), and when you click on the +, you then get an Add to Opera tab with suggestions (and an address bar where you can right an address for the Speed Dial item you want). (Alternately, you can drag a bookmark on the Personal Bookmarks Bar onto the blank square with the + and create a Speed Dial item.
As far as I'm aware, there's no way to eliminate that + in the empty square on the Speed Dial. It follows your last speed dial entry (unless you have nothing on the Speed Dial and then it's the first thing you see), and is there to indicate "click on me to create a Speed Dial entry, or drag something onto me to create it.