Designers put in lots of time and effort to give you the best designs possible. You need to be honest with them about what you like and what you don't like. Not giving timely feedback wastes both the designers efforts and your time. If you don't like a design at all, it is up to you to eliminate it from contension.
You need to choose a winning design and pay the designer the prize you promised. Don't give up on a contest. Designers will be more than to tweek their designs to match your needs.
If it's a guaranteed contest, Mygraphic Studio (MS) will pay the prize to the designer. If not, you are required to have PayPal payment as an option. However, you and the winning designer may arrange another form of payment if you wish.
When a designer uploads the files for the winning design, you have exclusive copyright. But before that, you may not use the design, nor any of the other designs entered into your contest. Copyrights of unpaid designs remain with the designers.