Please don’t cross-post the same question again with some hours difference in time only - what didn’t you like in answers given to you @ http://dynamicsuser.net/forums/t/26671.aspx ?
(except that you happened to post your question in wrong forum area, but that’s OK for newcomer)
Okay lets start with the basics, the majority of account entries in NAV are posted through posting groups. It is the clients decision how the posting groups are created and configured, therefore you are in full control of where it posts. You really need to understand the posting groups to get a basic grasp of NAV.
You can disable it by removing the accounts from the posting group. However you cannot disable the posting of gains/losses where they ocurr, therefore if you remove the accounts and a transaction creates a gain/loss then it will not post. So you can remove or repoint the posting, but you cannot really disable it.
Go to the currency card, you will see the accounts defined against each currency, you can change them but you need to be aware of the financial implications of this if you are live!
Calculation of Foreign Currency Exchange Gain & Loss are a must for Accounting, if you have transactions in Foreign Currency. So, it is very Important to understand the Accounting Impact even before you think of twisting the accounts or thinking of even disabling them in Navision.
Secondly, you cannot disable these accounts but instead you can only point to other accounts as AdamRoue mentioned.
Lastly, Currency Gains should not be pointed to Receivables as these are not income but gains made against Foreign Currencies. This is how I would view it. Please speak to your company Accountant or Auditor to understand more on Foreign Currency Exchange Gain & Losses.