A common task is to prevent jobs from running on a particular node by marking it offline with pbsnodes -o nodename. Once a node has been marked offline, the scheduler will no longer consider it available for new jobs. Simply use pbsnodes -c nodename when the node is returned to service.
Also useful is pbsnodes -l which lists all nodes with an interesting state, such as down, unknown, or offline. This provides a quick glance at nodes that migth be having a problem.
See the pbsnodes manpage for details.