C182 pilot reported a loss of control during landing after a passenger inadvertently activated the left toe brake resulting in a runway excursion.

Narrative

Landed at ZZZ Airport. I was taxing to exit on Taxiway 1 after given instructions from the Tower. I was rolling about 10 Knots when my passenger (First time in an airplane) decides to stretch his left leg and locked my left brake. Such action caused the airplane to Spin/turn 180' to the left into the ditch and off the runway. The airport operation was closed for 40 Minutes. A Mechanic was called; checked the airplane and all was good. We pushed the airplane off to the runway; started it and went to the restaurant transient where the Mechanic and I checked the airplane brake operating system. All was good.Before flying back home and after lunch; I tested the airplane brakes and all was good. We flew back home and asked my passenger to switch with another passenger and sit in the back. The airplane ran well including landing and braking. You may note that I called the FSS weather briefing 15 minutes before taking off from ZZZ1. I familiarized all passengers with seat belts functions; door locks and not to touch anything in the airplane during take off; landing and taxing. Especially; the front passenger to keep his hands; legs and knees off the rudders; yoke and etc... I did a preflight to the airplane and all checked out good.