ckbk includes a number of dietary filters - vegan, vegetarian, kosher and gluten-free, are accessible via the Diet pulldown on the search results page.
But a much wider range of exclusions can be incorporated into your search.
For any ingredient you wish to exclude, just prefix it with "-"
This also works for groups of ingredients, such as
e.g.
curry -onions (no scallions or shallots either)
soup -alliums (no onions or leeks or chives)