How to contain feed items from overflowing into next column

Question

I'm using WP RSS Aggregator, and I've styled the feed into containers, along with creating a 3 column layout. Limiting the feeds to 18 per page, I'm experiencing overflow issues. Setting the CSS to overflow:hidden !important; has not resolved the issue. Being the length of each feed item varies, I'm hoping for a solution that will carry through each feed category. Below is a link to the feed page, followed by the CSS I've implemented for feed imports into their respective pages. Thank you in advance for your help.

enter image description here

div.wprss-et-social-buttons {
        display: none;
    }
    .thumbnail-excerpt img{
        float: none !important;
    } 
    .thumbnail-excerpt {
        text-align: center;
    }
    li.feed-item{
      overflow: hidden !important;
        border: solid #000 2px;
        border-radius: 5px;
        padding: 10px;
        width: 250px;
        background: #eee;
    }
    .thumbnail-excerpt {
        overflow: auto !important;
    }
    wprss-feed-excerpt{
        font-size: 16px;
            text-align: center;
    }
    li.feed-item > a{
        font-size: 23px;
            text-align: center;
    }
    ul.rss-aggregator{
        column-count: 3 !important;
        list-style-type: none;
    }
    @media only screen and (max-width: 980px) { 
    ul.rss-aggregator {
        column-count: 2 !important; 
        }
    }
    @media only screen and (max-width: 479px) {
        ul.rss-aggregator {
        column-count: 1 !important;
        }
    }