Turn off/delete automatical break symbols

Right-click on an element. Select: All Similar Elements> Delete

Hmm, not sure what you mean by "automatical break symbols". MuseScore has Line Break, Page Break, and Section Break elements, but none of them are automatic - you have to placed them yourself (from the palette, or via keybaord shortcut, or via Edit / Tools / Add/Remove Line Breaks). And they definitely can be deleted by the Del key.

So I can only guess you mean something else, but I'm not sure. Maybe you are referring to "rests"? Deleting a rest and making no other change makes no sense musically - it leaves you with too few beats in the measure. So you don't delete rests; you simply replace them by something else - presumably, notes. Is that what you are are asking about? Or maybe you mean something else entirely. Anyhow, if you explain more what you mean, then we can give you better answers.