How can you stop the Octava (8va) effect?

When you place the octava symbol (upper or lower), its influence is to transpose one octave starting at the symbol position. If you want to limit its effect to a fixed duration (and not for the rest of the score), modify the options of this symbol. In the score, click this symbol while holding down the CTRL key. Click the "Full editor" button. Check the "Local change" box to the right of the dialog. On the left of the dialog, you will find 3 text boxes which let to determine the duration of the effect, in number of measures, beats and units (480 units for a quarter note). Click OK. The transposition effect will last as long as you specified it.