Copybotting is frowned upon in most virtual worlds. This includes OSGrid.
First, let's clarify what you mean by avatar. The name, the shape, the skin, the attachments?
1. Names, in this context, cannot be copyrighted. Trade names, such as a business name, can be trademarked.
2. Shapes can be created by various people that are very similar to others. Since this is a built in mechanism, it is not only possible, but very likely that two people created the same avatar appearance.
3. The texture files that make up skins can be copyrighted.
4. The attachments, ie, the textures and the scripts can be copyrighted. But nothing stops anyone from creating a physical shape, which isn't copyright-able.
Just out of curiousity, are they your avatars? Did you say anything to the admin who was present, or just expect them to do something? I personally don't pay much attention to random people, unless I am actually engaging that individual, and even then, how am I supposed to know that they are infringing on anything?
Is it possible that an SL user brought their creations to OSGrid? This happens very frequently, FYI.
No human being, regardless of your personal views, can possibly know about every avatar that has ever been created. To think so shows serious ignorance of the real world and life within it.
Talking of doing something is not infringement. Actually infringing is another matter.
The owners of any infringed copyrights need to send a DCMA notice to the infringing avatar. That is solely the responsibility of the owner of the copyright to take action.
The OSGrid admins aren't really in a position, without legal notice, to do anything.