DeniedGmane is a public, open service.
Actions that may lead to denial include:
It takes a lot to trigger a denial. Nobody who uses Gmane with a normal news reader can cause a denial.
Since there is no way to identify users, news readers are denied on a domain/IP basis. So if somebody on a machine close to you downloaded the news spool yesterday, you're likely to be denied today.
Here's a list of current blocked domains/IP ranges: