The Hadoop in Real World team shows us the proper way to remove a node from a Hadoop cluster:
This post will list out the steps to properly remove a node from a Hadoop cluster. It is not advisable to just shut down the node abruptly.
Node exclusions should be properly recorded in a file that is referred to by the property dfs.hosts.exclude. This property doesn’t have default value so in the absence of a file location and a file, the Hadoop cluster will not exclude any nodes.
Read on for more information, including what happens if you simply turn off the node.