Which version of mIRC are you using?
Please follow these steps:
1. Open the Address Book dialog (via the Tools menu in the menubar) and click on the Control tab, you should see a list of the users currently in your ignore list.
2. If the list is empty, add a user, and press OK in the dialog. Now open the Address Book dialog again to confirm that the user is still in the ignore list.
3. Exit mIRC.
4. Run mIRC.
5. Open the Address Book dialog again. Is the user still in your ignore list?