[otrs] Customer info problem

I have strange problem. I made a test ticket and now I can´t change customer info. If I go to zoom ticket => Customer => Set customer user and customer id of a ticket => I can put the CustomerID but if I put something to Customer User box, it clears all the boxes. So I can put only customerID, not customer user. Now when there is not any customer info, when I close the case, it always comes back to the system because there is no customer email, where the system sends the closing email. What can I do to get rid of this test case? -- Annika