I prepaid my room, but they charged my card $100 p...

I prepaid my room, but they charged my card $100 per night (total $300 for 3 nights). Then, we put a $50 room charge on the account. They helped themselves to an EXTRA $250 of my money! So, they held $500 ($550 hold, less $50 spent) for about ten days! NEVER going back! Since when, if they're already holding $300, does a $50 charge trigger an extra $250? Outrageous!