+1 - I come back to the use of simple software which requires a justification on a pop-up box when the fare / room rate, etc. exceeds the minimum by some given percentage. All management needs to do is scan through the exception items. Those who are "problem children" get a bit of counseling on expense management and those who keep it up either go away or not as the case may be.
This also allows flexibility to management to acommodate those they want to take care of. Who cares if somebody spends double the best fare if they're bringing $1 Mill. in business? Top performers get other perks, why not travel?