In order to archive a matter, you can make it inactive.
The only criteria to inactivating a matter is that all trust funds must be disbursed. 

To Inactive, go to the matter tab, locate the matter and click EDIT

At the bottom left you will see an Active checkbox. Uncheck this box and then save the matter.



This matter will then be moved to the inactive list.
To view this list, on the main matter page change the Sort filter to "Inactive"



To make a matter active again, simply edit the matter again and check the Active box, then click save.