Hello
What an interesting question! It got me thinking, and gave me an opportunity to explore Jonathan Green's Big Book of Being Rude (edited from the Cassel's Dictionary of Slang).
Green's highly readable selection of words is divided into several chapters, the heading most appropriate for your purposes would seem to be Stupid, Irritating and Contemptible. Here is a selection, I hope you will find them useful:
dicksplash (a personal favourite during my childhood in Canada, but has fallen into relative disuse in recent times)
the Irish tend to favour eedjit/eejit/ijit
My grandmother and fiancee (not one and the same) are both keen on "Daft 'a'peth". 'a'peth being an abbreviation of ha'peth.
You may also hear people being described as being "somewhat akin to a fool". Alternatively, suggesting someone is a "banana" is not likely to be taken as a complement.
I hope this short selection helps, for more information see
http://www.amazon.com/gp/product/0304368253/sr=8-2/qid=1140973198/ref=sr_1_2/002-0363286-3232878?%5Fencoding=UTF8
Matt