Maybe cabin crew should be to never use weight and balance as an excuse. Just state it's company policy instead.
I do the same as PoppaJo. Always give the passenger the option. The vast majority of the time the passenger does not travel. Sometimes they do. An important aspect of all this is this is will this passenger cause further problems? What is going to happen during the flight? Do you want to wear that risk?
As for weight and balance, the computer can put the aircraft right on the edge where one person moving will throw it out. Obviously there are safety margins which make it safe, but before departure if the computer says no then you don't depart.