Enable Group Policies to automatically logoff disconnected sessions or idle sessions after X minutes/hours
Not only does this reduce server resources (ram/cpu) that are used up by disconnected sessions and affecting the overall performance of the remote desktop server, automatically logging off disconnected and idle sessions may help lessen risk if vulnerabilities in Windows are found that allow a user on one active session to access another active session. See this post on how to enable policies to automatically logoff disconnected and idle sessions (highly recommended to do, and something we do by default now on new servers):