Solutions to deal with really bad users who enter incorrect data?

This question will be divided into two parts, the background story and the real question.

The history of back
I work for a consulting firm, and we have developed a piece of software that allows a company to manage their real estate. They rent around 1,000 apartments and houses, and the software tracks what is rented by whom, to whom it is owed and why, the properties and rental units that are owned are tracked, and the maintenance reports. It also manages billing.

So it's a pretty complex system.

The company for which we created it is committed to pay the minimum wage for their jobs and hires only women and does not require computer skills. Nor will they spend money on training new employees because they usually only last a few months.
Therefore, the software was responsible for having a "fisherman price" interface, and it should be as simple as the McDonald's interface. The only problem is that McDonald's probably spent millions on the design of their own, and McDonald's has a more simplistic business model and trained its users.

Despite all this, I feel that I have built a fairly intuitive interface, and I have had some guys in the office who do user tests for me. The results are good, I have fixed some things that I would not have seen due to my closeness with the software, but in general, all or almost all the tasks in the user's test were achieved without training.

!!!!! ¿¿¿¿! ¿¿¿¿¿¿¿¿¿! ¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿! ¿¿¿¿¿¿! ¿¿¿¿¿¿! ¿¿¿¿¿! ¿¿¿¿¿¿! ¿¿! Two months ago she changed about 50 tenant lease dates. I corrected it in the database and everything was fine for a while. Last month we received reports that the system was reassigning people to new homes, but it was she who was changing them. So I put a lock on that, so you can not change the tenants' house without clicking the big "Unlock" button on the image. That seemed to fix the problem. Then, two weeks ago, we got a report that the system (always the system failure) was changing the name of the properties. It turned out that the user thought he could search from the Property Name text box (labeled "Property Name", there is a button with a magnifying glass that says "Search" away from him). So now the way to manipulate properties is blocked in the same way that the tenant is. Yesterday, the system was changing the lease dates again, and the records showed that it was the same user. They want me to close the lease date as I have the name of the tenants' property and housing, 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 company) and will not be responsible for their lack of training and ability in their users. Actually, it would not solve anything either, until it was required to unlock each entry field before being able to edit it. They will not pay for the documentation, such as a user's guide or the help files that will be created, which is the only way I can see that it can be fixed.

The problem
A user without training is continuously entering erroneous data in meaningless locations (such as the telephone number in the license plate field). It eliminates or changes the critical data of the system that normally has to edit. The client has asked me to add an additional click to edit the data, but I think it will not solve the problem, just make it move to another field.

What can I do to avoid this bad entry that is not significantly different from good information?