How do I stop patients from leaving the hospital

I have the same problem. At first it didn't matter if they left, I'd just focus on treating the patients I already had, but now my doctor is level 5 I get told off by the game every time somebody leaves. I can't spend all day admitting people or I'll never get anything done!

I don't think it affects the career performance though so at the moment I just carry on ignoring them and working towards my goals instead.