bbPress.org

It should be this

@media and (min-width: 992px) {
 .bbpress .col-md-8 {
    width: 100% !important;
 }

 .bbpress .col-md-4 {
    width: 0 !important;
    display: none !important;
 }
}

You can also try it without the media queries.

.bbpress .col-md-8 {
    width: 100% !important;
 }

 .bbpress .col-md-4 {
    width: 0 !important;
    display: none !important;
 }