Vending Machine Bank

I recently got a Vending Bank that you put money in and press a button (1-6) and get candy. The candy dispenses from the holder (small holder mabye 1 inch wide 2inch tall) and it drops down to a slot in the bottom. None of it is motorized in anyway. You can put pennies, nickles, dimes, quaters, and dollar bills in via a slot that you put the bill in and a dial that you spin until the money goes inside the bank. I want to know if any of you can figure out a way to modify it to where you cant get candy until you put at least 25 cents in there because currently you dont have to put money in to get candy. If you need pictures i will gladly supply them upon request. Any help will help! Thanks!