Code:
DELETE FROM xf_ip WHERE ip = UNHEX(LPAD(HEX(INET_ATON('1.2.3.4')), 8, '0'));
Change 1.2.3.4 as needed.
I haven't tested this yet, so I'd recommend using the SELECT version to confirm it's grabbing what you expect and, as usual, I'd recommend backing up before running this.