How to prove to the client that our program is working properly?

Question

Work organization

We are in the business of automating stores.
We have more than 500 clients in the CIS. In general, everything is fine. 2 months ago, a client bought a program from us.
Now he periodically tells us that goods disappear in the program. In our program, goods do not disappear (otherwise we would be bombarded with complaints).
When we ask what specific goods are missing, he cannot name. When something says and names, we check the problem (restore backups to the required dates) and find that the product either did not disappear or was not originally listed.
In addition, the product may disappear if one of the users deletes it intentionally or accidentally.
The client is still sure that the problem is in our program, it is not possible to prove the opposite to him. He doesn't know how to formulate problems.
And in general, he believes that we constantly have to prove to him that the cause of the problems is not in us (and this takes time), to check problems that are not confirmed.
What should we do?

Answer 1

I suppose user actions are not logged by your program? This would solve the problem.
In your case, you need to look at the EULA and / or the agreement - are you obliged to respond to unreasoned requests from users at all?

Answer 2

not to work with assholes
not to delete, but to mark as remote
logging in the database
...
PROFIT!

Answer 3

How do you know in advance the asshole will be a client or not an asshole? And after 2 months I don’t want to return the money (so much time was spent on it).
In addition, when there is nothing to eat and you will work with potential assholes. As soon as the negative effects are minimized...

Answer 4

Logging fasten on all similar operations (adding-removing-changing goods).
Well and optionally - cancellation of actions.