well i've been flying for a whie now and really it is ground handlers responsibility to ensure the a/c is loaded correctly, if this isn't the case it is often left to us as they don't have to deal with it then.
in addition the requirement for abp's to sit in rows is only for landing and t/off, therefore unsuitable's are permitted to occupy the seats during the cruise.
a technique i have sometimes employed for those of a particularly difficult disposition is normally to ask the fittest young man travelling with his mates (boys traveeling with girls are sometimes willing to with a sly wink
), offer him a free drink (the bar never used to add up anyway before i left charter) or first service and ask him to sit there for take off and landing and swap for the cruise. it normally worked and a flutter of my big brown lashes normally helped!!!