Actually, it does ^_^.  According to eBay policy,
if you email a bidder telling them not to bid on
your auction and then cancel their bid they cannot
bid again.  That would be a "bidding offense" 
as listed clearly on the SafeHarbor investigations
page (as well as elsewhere on the eBay site).
According to eBay, if you send them a copy of your
email to the bidder, they can procede to suspend
that bidder (assuming he has bid again).

I don't know what you can do about last second 
bidders, but you could always email them before 
the auction asking them not to bid, and that 
should be enough.

Keys