This is annoying me as well.
I keep getting the same books recommended even though I already own them! Of course I don't expect Kobo to know what I have purchased from elsewhere (and infact I think this means their recommendations are pretty good) but once I say I have already read it - that means I have ALREADY READ it! So repeatedly recommending it is just wasting the opportunity to show me books that I might want to buy.
However even more infuriating is that some of the books Kobo keeps recommending for me I have actually PURCHASED from Kobo!!
For example: The Reversal by Michael Connelly - purchased Aug 18 2011 - removed from my recommended list for the third time this morning.