Thanks to you all!
It really depends on how offensive and nasty you want to be. The number of possible terms depends only on your imagination.
"Clueless moron", "pointless retard", "pea-brain", "insignificant loser" - these are all pretty much guaranteed to be offensive.
Wow! Many thanks! And about this word loser, does it mean the same as 'They can't do anything for toffee?