Can someone tell me how to block someone?

I have been getting a lot of incomprehensible comments (which contain link to an external website) to my old posts from this person.  I don't want to move to a comment by approval system or make my posts neighbourhood only at this stage - I just want to block this one person.  Anyone know how to do this?  Have also contacted vox about it but just got a generic response back from them so not sure if they will do anything about it.  Spam and porn really do seem to be getting out of control on vox.

Edited to add: After reading the vox help section, I have now also edited my settings that only comments from neighbours go through automatically.  This is probably a sensible thing to do anyway.

Edited again to add: Just got this response from vox.  :)

Hi,

Thanks for reporting this. The account has been removed from the system. You can delete any spam comments that may have been left on your blog. We appreciate your bringing this to our attention.

The Vox Help Team