You can change default site layout anytime. 

Navigate to omniSite \ Site Layout and choose new layout. Configure it and click Save as application default.

Disable user ability to change site layout

By default users are able to change their favorite site layout. You can disable it.

Go to User Roles under omniCustomers \ Roles.

  1.  Select edit User role. 
  2. Navigate to App Permissions section.
  3. Make sure Visual Settings permission is disabled for the user role.