To close (archive) a case, in the "Cases (Projects)" tab, select the case in the left side panel, click on "Case Details/Edit",  click on the Archive button on the right side. The closed case will not appear in the Cases (Projects) list.   
 
To Re-open a closed case, click on the "Cases (Projects)" tab, in the search box, enter the name (or a few letters contained in the case name or enter %) of the case, select the case in the left side panel, click on "Case Details/Edit", click on the UnArchive button on the right.