I find that a brief explanation works wonders in cases such as this. As was stated earlier, many pax are 1st time or irregular flyers, they are truly not aware of the responsibilities of sitting in an exit row.
A comment such as: "After you get yourself settled, just ensure that all your luggage is in the locker, we have to keep the emergency exits clear, just in case we need to use them."
or: "I'll pop that bag up in the locker for you for the takeoff/landing."
Statements with an explanation are not requests.
People don't often argue with things that are presented as a fait accompli.
My airline has a policy of only allowing able-bodied passengers to sit in exit rows. Mistakes occasionally occur at checkin, but an explanation (and a smile) seems to work a lot better than officiousness.