Like Anno2 suggested, blocking IPs would be the easiest and probably most effective route to take. I have had to deal with spammers in the past, mostly from India. They wouldn't come in frequently, just every once in a while. Nevertheless, I look up their IP address and block the range from whatever ISP they are using. Since they were in India and I do not have any members from India of which I am aware, I felt this was a safe step to take.
Anyway, have a look at the offender's IP address(es) and see if they are all the same or similar (if you still have them on file.) I would imagine they are, and in that case you can probably safely block the IPs from your control panel.
Let us know how things work out, or if you need further assistance.