Welcome to eBay.

Rather than posting the link to the area on eBay that lets you block buyers I will let you know how to get there.

At the bottom of every page there are some links.  Click on the 'Site Map' link.  The site map is a great place to get lots of information and advice.  Anyway, in the centre column there is a heading "Selling Activities".  The second item under this heading is the "Block buyer/bidder list".  Click that liink, when the page loads you click the "Add an eBayer . . . " link and then type in (or copy and paste) the ID of the eBayer you want to block.  Then click submit and it is done.

Sellers can only leave a +ve comment for buyers.  You have the option of leaving no feedback.  If you leave a +ve with a comment that conflicts with the rating (i.e. a false +ve) then you will get an eBay policy violation if the buyer reports you.  Better to do the UnPaid Item case so that the buyer will not be able to leave you feedback.

Come back here to the boards if you have any other questions, especially if you are unsure of a situation or just want to make sure you avoid policy violations.