How do we mark an exchange rate as inactive

Hi,

I wonder if anybody could offer some guidance on my question below:

We have some exchange rates set up historically that may have been used in the past.  However, these are no longer used and therefore the rates are not kept up to date.

When somebody now comes along and enters an expense line in and puts a currency code in of these old currencies, the system uses the latest effective date it has and converts the amount using that exchange rate, even if that is 10 years old.

How do we clear up unused exchange rates ? 

We don't appear to be able to put an end date on them.