It is possible to block specific emails when people try to add new links, but as Eliot has mentioned, it's better to block the IP address.
If you think about it, anyone spamming your Links site could easily make-up any number of false email addresses to get past a checking system and you'd probably spend most of your time managing which emails to block and still not block much of the spam.
I had a similar experience where someone had copied my add.cgi form and once I'd added the modification to block their IP addresses all the spam stopped
You'll find details of the IP Blocking Mod in the forum here: http://www.gossamer-threads.com/...qango%20spam;#151544
Hope this helps.
All the best