I would check what the agent/landlord means.
An ‘openhouse’ isn't usually ‘just wander in off the street’ - the agent has viewers details - because they want to be able to contact them to follow up.
Usually the agent is present.
I liked ‘open house’ - it got through all viewings in one clean! 😀 Much preferred to ‘ there is a viewing on Friday…clean and go out, a viewing on Tuesday...clesn and go out, a viewing Thursday….you get the picture.
One afternoon if open house, flat let, job done!
Or as above, refuse ( unless he wants to relet with the same agent)