I recently saw this question posted on the MS CRM Forum and I can clearly understand why this would be a requirement. When you look at fields using an advanced find you get a list of all attributes associated with the entity. This sometimes can be confusing for end users.
In the screen shot below you can see a list of all fields associated with the account record type.
To remove fields not used on the form you will need to set the Searchable field to No on the attribute customization form. To do this you will navigate to the Settings area, click customizations, click the attributes tab and then click on the attribute you want to remove from the list. You will then need to set the searchable field to No.
The screen shot below demonstrates how do do this. You will then need to publish your changes.
Once the changes have been implemented the non searchable field will not be visible in the Advanced Find field list.
Be advised that making a field non searchable will also make the field not visible in the report wizard.