I took a read through the SMC Manual (SMC User Manual) and sadly there is no way to reserve DHCP addresses so the phone could get a different IP when it connects however the lease time Defaults for 1 week so it should always get the same IP unless it is off the wireless for more than a week.
You can do blocking by IP on Page 69 using access control. You would put the service as any or all and the single LAN IP you want to block access from.
The other option is MAC filtering(as GDKitty suggested) on page 59 of manual. It looks like it can learn the MAC addresses of all the clients so easy to add them to the list. You would add all the MAC addresses you would like to allow and do not add the phone. You would then change the MAC Filtering mode to allow so only the configured MAC addresses would be allowed.