Omniweb can use regular expressions to filter banner ads, or even whole web pages. You can find it in the Preferences, in the Privacy section. There are a few examples to work from, but the gist of it is:
.*\/ads\/.*
The above will match any URL that has /ads/ in it. This should match a lot of advertisements. The .* means "anything." The \/ will match a "/", because just using a "/" has a special meaning for the regular expression. Be careful, though. You could block normal web pages with this feature.
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