[header] How to tell someone you can't cash their check [title] Explain to the customer, that because they do not have an account at the bank, you are unable to cash their check. [step] Do this in an apologetic manner, by talking in a calm, empathetic voice. [title] Wait for the customer to respond. 
[step] Often customers may become upset and ask why. This is when you must explain to them that it is the bank's policy.