If you want to disable the app for a time being, go to the Voila dashboard and disable it from the App action.

If you want completely remove Voila App code:

Step 1:

In theme.liquid File : Find {% section 'ms-voila-sec' %} and remove one line code.

(you can find below of the theme.liquid file and before end body tag(</body></html>) )

Step 2:

Delete ms-voila-sec.liquid file which is in Sections.