You can give designers feedback in a number of ways:
Rate their designs. This gives designers an indication of what you do and don’t like. Just roll your cursor over the stars below each design thumbnail image, and click the rating you’d like to give. Early in the contest, it’s usually a good idea to not rate designs above 3 stars—this keeps your contest competitive. The 4- and 5-star ratings are the highest possible ratings that you can give, so new designers might be discouraged from entering a contest where designs are already getting ratings at that level.
Eliminate designs from the contest. This lets designers know that the specific design wasn’t what you were looking for, as well as the reason it’s not being considered. To eliminate a design, simply click the gray X icon that appears below the entry, and then select whether the design was Off brief, Not to my taste, or Not original.