Unwanted comments are not “spam”. Do not mark his comments as spam and confuse Akismet unless he is posting stuff like selling drugs and/or porno, etc.
Moderate your comments and delete all of his. Eventually he will give up submitting them.
You can blacklist his IP but I doubt that it will do any good at all because he will just get another one. -> Options-> Discussion
scroll down to the blacklist box and enter the information and then click “Save Options”