This question I'm going to break into two parts, the back story, and the real question.
The Back Story
I work for a consulting firm, and we've developed a piece of software that allows a company to manage their real estate. They rent out around 1,000 apartments and houses, and the software tracks what's rented by whom, who owes what and what for, it tracks the properties and rental units owned, and maintenance reports. It also manages the billing.
So it is a fairly complex system.
The company we created it for makes a point of paying minimum wage for their jobs and hires only females and does not require computer literacy. They also won't waste money on training new hires because they usually only last a few months.
Therefore, the software was commissioned to have a "fisher-price" interface, and it should be as simple as McDonald's interface is. The only problem is, McDonald's probably spent millions designing theirs, and McDonald's has a more simplistic business model, and they train their users.
Despite all this, I feel I have built a fairly intuitive interface, and I've had some guys in the office do user tests for me. The results are good, I've fixed a few things that I wouldn't have seen because of my closeness to the software, but overall, the achieved all or nearly all the tasks in the user test without training.
However, there is one user in particular at this company that cannot figure out how to use the system. Two months ago she changed around 50 tenant's lease dates. I corrected that in the database and all was well for a little while. Last month we got reports that the system was reassigning people to new living quarters, but it was her changing them. So I put a lock on that, so you can't change the tenants living quarters without clicking a big "Unlock" image button. That seemed to fix the issue. Then two weeks ago we got a report that the system (always the system's fault) was renaming properties. It turned out the user thought she could search from the Property Name textbox (labeled "Property Name", there is a button with a magnifying glass that says "Search" far away from it). So now the form to manipulate properties is locked in the same way tenant's is. Yesterday, the system was once again changing lease dates, and the logs showed it was the same user. They want me to lock down the lease date the way I have property name and tenant living quarters, but I think it has gone too far.
If I make the change they request, they will continue to blame the software (and the consulting firm) and not take responsibility for their lack of training and skill in their users. It also wouldn't actually solve anything, until every input field was required to be unlocked before it could be edited. They won't pay for documentation such as a user guide or help files to be created, which is the only way I am seeing that it can be fixed.
An untrained user is continually entering bad data in nonsensical locations (such as phone number in the license plate field). She deletes or changes critical system data that she normally has to edit. The customer has requested I add an additional click to editing the data, but I feel that won't solve the problem, just make it move to another field.
What can I do to prevent this bad input that is not significantly different from good data?