Yeah, like Nisto said, we support the minus (-) operator to exclude keywords in advanced search. So here's a search for doujin albums without "ZUN" in the composer field:
http://vgmdb.net/search?do=results&id=77031
Note that if you use simple search (i.e. the search on the top left navigation) you can't have only excludes (such as "-soundtrack -original") -- you must have at least one regular keyword.