To hide only certain calendars, go to the settings screen, tap "Calendar Settings" → "Visible Calendars", and uncheck the calendar. It will no longer be shown in Smart Planner (the event data itself is not deleted).
If you don't want to see any Google Calendar events at all, turn off "Calendar Settings" → "Link Google Calendar" in the settings screen. Turning the link off never deletes events in Google Calendar or events in the "Local calendar".