Gabriel does not have a box to check "Do Not Mail".  A workaround would be to go to Family Search and open their family record and click on the Mailing Address Tab and fill out as below.  You could use the date fields if this was a temporary situation or if the family does not want to receive mail over the summer etc.  It will print a label without any address and still maintain the family address information in the Family Record.  If at any time they want to receive mail again simply go and uncheck the box in Mailing Address Tab.