Clear for an entire domain:
varnishadm "ban req.http.host == www.mydomain.com"
Clear for a URL on a domain that starts with something, or ends with something (just regex, so change as you like):
varnishadm "ban req.http.host == www.mydomain.com && req.url ~ ^/some-directory"
varnishadm "ban req.http.host == www.mydomain.com && req.url ~ .css$"
Clear for a specific URL:
varnishadm "ban req.http.host == www.mydomain.com && req.url == /some/specific/url"
Here is a nice cheatsheet for ya - http://kly.no/varnish/regex.txt Show archive.org snapshot