The maintenance functionality in Zabbix is aimed at a well-planned environment where maintenance is always planned in advance. In practice, people often want to place a host in maintenance quickly and then simply remove it manually a bit later. With all the periods and other things maintenance entry requires, it's not quick enough. A slightly hackish workaround is to create a new host group (ex-maintenance) and a maintenance period that is always active (make sure to set its end date far enough in the future). Include that host group in the maintenance entry, and then adding a host to the chosen host group will add that host to maintenance. Of course, you will have to remember to remove the host from the host group afterwards. Another workaround is to use the API.