Sometimes you need to do maintenance on a XenApp server via RDP but you want to keep users from launching XenApp sessions. If you disable logins, you disable RDP logins as well as ICA logins.
An easy way to work around this is to create a custom Load Evaluator. Use the Scheduling rule but do not add any time to the rule. This will make the server unavailable for Load Balancing but still allow you to RDP into the server. This works on XenApp 6 and older.