Problem with key change

If you do not want any Naturals (you called them becas) you can use a section break rather than a system break. This will treat the new line as a new section or movement and not create any courtesy key signature. You will probably want to right click the system break, choose system break properties and adjust the options to make it work like a system break.

A couple of different ways. First, though, I'd point out what you are doing is a little dangerous, because it makes it looks like that C arpeggio still has the B major key signature. Maybe people would figure it out from the context, but still, it would bug me. I'd put the C example first to avoid that potential confusion. but anyhow, to answer your question more directly, as I said, a couple of different ways:

1) Disable courtesy key signatures for the score globally in Style / General / Page.

or

2) Select the clef at the start of the measure in C and press Ctrl+Alt+Shift+Right, the shortcut to move to the next element. This will select the non-displaying C major key signature, as you will be able to tell by looking at the status bar. Then you can disable courtesy key in the Inpector.

You'll still get the C major key signature with naturals, but you can make it invisible and then close up the gap using a negative "Trailing space" in the Inspector. It's a hack, but again, this is not a notation I'd recommend in the first place. As long as we're talking hacks, though, you could consider leaving the key signature B, then hiding the resulting naturals on the notes, or just notating it diatonically using the Inspector to adjust the pitch for playback.

EDIT, ooh, just saw Mike's much nicer hack, I take mine back :-)