To close (archive) a case, in the "Cases (Projects)" tab, select the case in the left side panel, click on "Case Details/Edit", Check the "Close This Case" checkbox and Save. 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) of the case, select the case in the left side panel, click on "Case Details/Edit", uncheck the "Close This Case" checkbox and Save.