i had simular problem with a buyer, left him neutral feedback, NOT a wise thing to do :S
i'm not aware that what you want is possible, but they could make it with filters, just like with the block option, that a certain buyer can't bid/buy on your auctions, then again, that would already rule out that that particular buyer would then buy from you, if you want to leave manual feedback, that's always possible.
they could make a sort of '' stop automatic feedback '' and then activate it again, i think that that is already possible here
activa feedback & de-active automatic feedback, look in the options.
http://www.auctiva.com/sales/landing.aspx
http://www.auctiva.com/account/feedbacksettings.aspx
Enable Auto Feedback: select that part for automatic feedback & unselect it for manual feedback
with selling manager pro you can select it to paypal settings...
when paypal payment has been recieved, leave feedback is 1 option
and the other is when paypal & feedback has been recieved, leave feedback
hope this helps