Or transferring your money into their internet banking account?
Really nothing but honesty and a risk of jail time. My parents had $120 taken from their banking account in this way. They paid for meds using a check. The person behind the counter copied their routing information then created eChecks (can't remember the website) and paid off her cable bill and telephone bill. They ended up getting their money back from the bank after a week or two and the women, as far as I know, was never prosecuted. The amount taken was too small for the effort to take her to court. Hopefully she got fired at least.