In the Address Book, when you click Tools>Options, you should see an
Addressing tab with three fields.  The top field is a drop down labeled
"Show this address list first".  The second field is a drop-down labeled
"Keep personal addresses in".  The third field is labeled "When sending
mail, check names using these address lists in the following order".  In
that list you can select the folder you don't want and click Remove.

If that doesn't produce the desired results and you are using Outlook
2002/2003, in Outlook's main window click Tools>E-mail Accounts>View or
change existing directories or adddress books>Next.  Select Outlook Address
Book and click Change.  Select the contacts folder you don't want and click
Remove Address Book.