How can I report a suspected copyright or trademark infringement in a repo-hosted application?

First thing: This is up to the copyright holder to lodge any complaint. You can't do anything because it's (sincerely) none of your business.

If they feel there's a valid complaint, they can contact Canonical (see the bottom for a phone number). A DMCA notice is pointless because it's the wrong jurisdiction, but I'm sure they would listen to sense.

I would imagine the owner should also be taking other civil action against the other project if they feel there has been an infringement.

But as others have said, this is how open source works. If something doesn't work for you, GPL encourages you to fork it and fix it. The basic rules are you just don't use the same name but all that really depends on the license. That appears to be what has happened here... But similarly, this is for the owner to interpret.

Having seen Flavio's post, I'd like to make a few more comments:

• Canonical should not be listing this as a proprietary download. You're right there.
• Most non-opensourcerers might think blatent copying is evil but they've changed the name and they're working on your code just in their space. I can't see how people would get the two names confused either.
• You can pull back fixes into your project from theirs. That's the beauty of GPL - even though they've added code, it needs to be the same license as the original.
• However, there is a huge problem: Stroget has re-licensed your project as LGPL3. LGPL is more permissive than GPL so it's not compatible. They're breaking your license to them.

The relicensing is the only thing you can think to action here (IMO, but remember: IANAL) but it is a significant issue. Talk to the project leaders or contact the FSF.

I'd like to point out that I never talked about copyright infringement.

Of course that guy broke every single rule in open source etiquette by not giving back his "bug fixes", pretending "Stroget" is a new project while it doesn't contain any changes and basically behaving like he was the original author. Still the GPL allows it.

One minor issue is that the Software Center displays the "Proprietary" label.

I asked Canonical whether it is their policy to allow a renamed build of a package in their store. I asked if I could rebuild and rename Nautilus and get it accepted. I'm waiting for their decision.