There are plenty of grocery stores and supermarkets in Ružomberok, and you can organize your own menu if you want to. For example, you can go to the Kaufland supermarket. They accept credit cards without any problems. If you want to arrange for food to be delivered to your room or address, Ružomberok has that option.
Pizza delivery in Ružomberok is offered by pizzeria da Andrea u Taliana and Ristorante Camino. Chinese and Thai food, popular in Europe today, will be hard to find in Ružomberoka, as will sushi delivery. But you will find many restaurants with European and Central European cuisine, meat and vegetables are popular.