[THIN] Reboot active app server

I need to reboot an active app server in my farm.  It has active
sessions.  I don't need to do it immediately but want to do it soon.  Is
the best method to do this simply to disable ICA protocol so no further
sessions can connect and wait until users finish their active sessions?

I have app 2 servers in the farm where all apps are published, will
disabling ICA on one cause them to get any type of error when they try
to connect or will they simply hit the other server until I re-enable
ICA connections?

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