Is there a way to block a particular address from downloading one or more torrents?

I think the IP filter can be made to block specific IPs by creating your own blocklist file.  That is covered here: http://support.tixati.com/IP_Filter_View

For individual torrents if you right click on a peer in the peers view you can choose "Ignore" and effectively block them, but they would have to be already in your peers list first for that to work.

You can right click a peer and select Ignore from menu but it's temporary and just for one torrent. Other possibility is to use IP Filter, it can be enabled in settings. IP Filter requires a list, put the address in a text file formatted like this:

description:IP Address-IP Address
remember to leave an empty line at the end of the file. Save the file and add it as a list.

I was wrong, Ignore peer function affects all torrents, it is temporary though, works until Tixati is closed.