'managed' vs 'unmanaged' in the context of USM has nothing to do with maintenance mode, or suppression of alarms, it indicates whether the ACE probe is allowed to configure the machine automatically and does nothing more.
In some cases you might discover a huge network but only want to manage (i.e. monitor) certain machines, so the default behavior is that the ace probe will not apply a monitoring template to any 'unmanaged' machines. There is a setting in USM, 'treat unmanaged machines as managed' which would cause the ACE probe to apply monitoring templates to all machines in a group whether or not they were set as 'managed'.
If you have already set a machine as managed, and then configured it with a template, setting it to unmanaged will basically have no effect on the current monitoring. It will tell ACE not to automatically configure this machine anymore which might then result in unexpected behavior when you go to remove the monitors and ACE skips this machine since it's not managed.
If you are looking for maintenance mode this could be set in the Maintenance Mode portlet and not USM.