That should not happen if the ILS is left to auto-tuning. Once the approach is loaded via the DEP/ARR page, the ILS for that runway will be tuned, if available. There should be no conflicting ILS receivable in the terminal area.
Our procedures include deleting any manually tuned NavAids when climbing through 10,000'. If that is not a procedure at your airline, you may consider making it a personal one...
I have not tried manually tuning an invalid ILS before attempting an IAN approach, so I cannot confirm that it happens as you say. I HAVE loaded an RNAV approach and allowed it to automatically tune and capture the ILS for the same runway. Works fine.
Last edited by Intruder; 18th Feb 2013 at 19:54.